Compact MEMS external cavity tunable laser with ultra-narrow linewidth for coherent detection

Di Zhang; Jianyi Zhao; Qi Yang; Wen Liu; Yanfeng Fu; Chao Li; Ming Luo; Shenglei Hu; Qianggao Hu; Lei Wang

doi:10.1364/OE.20.019670

1. Introduction

Transmission systems with ultra-high capacity and spectral efficiency at bit rate of 100Gbit/s or beyond are highly demanded to meet the requirements of fast growing internets. Multi-level modulation formats, such as M-ary phase-shift keying (PSK) and quadrature amplitude modulation (QAM), fueled by coherent detection are proposed to be an excellent solution [1]. For example, polarization-multiplexed QPSK (PM-QPSK) has been regarded as an implement agreement by the Optical Internetworking Forum (OIF) for commercial 100Gbit/s dense wavelength division multiplexing (DWDM) transmission [2]. Recently, coherent optical OFDM (CO-OFDM) modulation scheme has attracted extensive attentions. It enables Tbit/s transmission over thousands of kilometers using band-multiplexing [3–5]. However, one of the major obstacles in coherent detection is the phase noise of transmitter and local oscillator (LO) lasers [6]. In a 40Gbit/s system using classic Mth-power phase estimation [7], theoretically gives that the limitations of laser linewidth leading to 1-dB penalty at bit error rate (BER) of 10⁻⁴ are 4.4MHz, 330 kHz, 50 kHz and 12.5 kHz for QPSK, 8-PSK, 16-PSK and 16-QAM, respectively. What’s more, the laser linewidth constraint will become more stringent for denser constellations, longer transmission distance as well as higher symbol rate [8, 9].

The external cavity tunable laser (ECTL) consisting of gain chip and separated bulky optical filters, due to its narrow linewidth and large tuning range, has been widely used in the coherent lightwave transmission experiments with multi-level modulation implementing both single carrier and multi-carrier schemes. However, the application of ECTL is limited because of its relatively large size, this disadvantage becomes more severe when ECTL is used in the high-speed ultra-long haul optical transponder. NEC reported a compact ECTL based on a liquid crystal mirror and an intra-cavity etalon with high wavelength accuracy [10]. However, the linewidth of the ECTL is about hundreds of kilohertz [11] which limits its application in higher-order PSK or QAM coherent systems. The implementation of microelectronmechanical systems (MEMS) technology introduces significant improvements in constructing the external cavity of a tunable laser, such as total dimension, tuning speed and mechanical stability [12]. In [13], a MEMS ECTL for telecommunication use with linewidth of 15 kHz is presented, but the disadvantage of this MEMS ECTL is the complex MEMS actuator which forces the mirror to rotate along a remote virtual pivot. Although many attempts focus on the continuous tuning range of MEMS ECTL without mode-hop [14, 15], this may not be the critical issue in telecommunications where wavelength accuracy and compact size need to be paid more attention to.

In our work, an ECTL based on a single-axis MEMS mirror for coherent optical communication is numerically and experimentally demonstrated. A hybrid analysis method which combines simulation of free-space external cavity by Gaussian beam propagation with the help of ZEMAX, and simulation of semiconductor active region by time domain traveling wave (TDTW) method, is used to design and optimize the device. The effects of components' parameters in the external cavity on performance of ECTL including wavelength accuracy and tuning characteristics as well as spectral linewidth are shown. This ECTL can be placed in a butterfly package compatible with OIF’s standard, a wide wavelength tuning range about 40nm in C-band with a narrow linewidth of less than 50 kHz and wavelength accuracy of ± 1GHz over the entire tuning range can be obtained. When applied to coherent detection, both the block length in single-carrier QPSK and fast Fourier transform (FFT) size in OFDM systems are highly related to the laser linewidth tolerance [16,17]. The performance of this device is then evaluated in single-carrier PM-QPSK and multi-carrier OFDM systems, the results are perfectly matched with numerical simulations.

2. Configuration of the MEMS ECTL

The schematic layout of the ECTL is a modified Littman-Metcalf structure as depicted in Fig. 1 . The ECTL consists of a semiconductor gain chip of 1000μm length with typical beam divergence of 16° (lateral, x-direction) and 30° (transverse, y-direction), a collimating lens, a fused silica etalon with free spectral range (FSR) of 50GHz, a diffraction grating with line density of 1030lines/mm and diffraction efficiency about 90% for P-polarization, and a single-axis MEMS mirror. The direct-reflected zero-order beam detected by a monitor photodiode is used for wavelength locking with the help of the etalon which is similar to [10], and the details of the wavelength locking operation will be discussed in section 3. The single-axis MEMS mirror is a commercial product widely used in optical components such as optical switch and variable optical attenuator. It features simpler mechanical structure and lower cost compared with that in [13] as well as high reliability. The gain chip has facet reflectivity of 10% and 0.001%, respectively. The etalon’s transmission peaks are tuned to match the standard ITU-T wavelengths with 50GHz grid, and it also acts as a filter to suppress the neighboring modes of the lasing mode. The combination of the diffraction grating and the MEMS mirror constructs a tunable optical filter (TOF). This TOF, combined with the etalon filter, can select different ITU-T channels and make this ECTL tune over C-band with 50GHz as depicted in Fig. 1(b).

Fig. 1 (a) Layout of the proposed MEMS ECTL. (b) Combination effect of etalon transmission and TOF to realize a C-band tunable laser with 50GHz grid fixed at ITU-T wavelengths.

Download Full Size | PDF

3. Simulation and design of the MEMS ECTL

Considering there are several discrete micro-optics components in the external cavity, the design of this ECTL should compromise among various parameters and make a trade-off for a given target. In this section, we use a hybrid simulation method to analyze the characteristics of the ECTL in two steps: for the passive external cavity without considering resonating behavior, ZEMAX and Gaussian beam propagation method are implemented to calculate the impact of the specifications of collimating lens and MEMS mirror on the passive external cavity characteristics such as TOF bandwidth and cavity loss. Then we use TDTW method to demonstrate the relationship between wavelength accuracy and etalon specifications as well as the tuning characteristics of the ECTL. The spectral linewidth of the ECTL is also theoretically discussed. Based on the simulation results, an optimal ECTL design for coherent detection can be obtained.

3.1 Simulation of the passive external cavity

The collimating lens should be chosen by considering both coupling loss of the external cavity and bandwidth of the TOF, typically an aspherical lens with large numerical aperture (NA) is required. What’s more, a large collimated beam size will produce a narrow TOF bandwidth which is expected to maintain good channel stability as shown in Fig. 1(b). From the aspects of Gaussian beam coupling, wavelength-dependent diffraction angle will cause wavelength-dependent coupling efficiency $η (λ)$ , using Eq. (1)

η (λ) \propto \exp {\frac{- 4 π^{2} ω^{2} \cdot θ {(λ)}^{2}}{λ^{2}}}

Where $ω$ is the collimated beam waist perpendicular to the grating lines, and $θ (λ)$ is wavelength-dependent diffraction angle. We first calculate and measure the relationship between the TOF bandwidth and collimated beam waist, as shown in Fig. 2 .

Fig. 2 Calculated and measured relationship between collimated beam size and TOF bandwidth.

Download Full Size | PDF

It is found that in order to keep the 3dB and 20dB bandwidth of the TOF less than 0.8nm (two times of the etalon’s FSR), the collimated beam waists should be larger than 0.3mm and 0.77mm, respectively. Thus an aspherical collimated lens with NA of 0.6 and effective focal length of 1.49mm is used, with the help of ZEMAX, the ray tracing in the external cavity can be performed as shown in Fig. 3(a) . The use of ZEMAX provides a precise simulation of beam size at different positions by taking all the optical surfaces in the external cavity into consideration. The waist radius of collimated Gaussian beam are 0.654mm (x-direction) and 0.733mm (y-direction) with Rayleigh lengths of 869mm and 1092mm, respectively, indicating a good collimating performance and a low coupling loss which have a close relationship with linewidth suppression [18], thus the beam size at the MEMS mirror are nearly the same with the waist size. Using Eq. (2)

Fig. 3 (a) Ray tracing diagram by ZEMAX. (b) Simulated cavity loss induced by limited MEMS size.

Download Full Size | PDF

α (r) = \frac{2}{π ω_{x} ω_{y}} \int_{0}^{2 π} \int_{0}^{r} \exp {- \frac{2 r^{2} \cos^{2} (θ)}{ω_{x}^{2}} - \frac{2 r^{2} \sin^{2} (θ)}{ω_{y}^{2}}} r d r d θ

We can calculate the loss $α (r)$ induced by limited MEMS mirror size. Here, $ω_{x}$ and $ω_{y}$ are the calculated beam waists in x- and y-direction using ZEMAX, respectively, r is the MEMS mirror radius. The simulated result is shown in Fig. 3(b).

Typically, larger MEMS mirror size will result in smaller round-trip loss of the external cavity. However, due to the MEMS fabrication, large mirror size always leads to bad mechanical performance and high cost. Considering both reliability and availability of the MEMS mirror, a commercial-available MEMS mirror with a radius of 0.5mm which can be rotated by ± 3° with an actuating voltage of ± 40V is preferred, thus the loss caused by limited MEMS size is about 1.89dB and the corresponding 3dB TOF bandwidth is 0.598nm.

The total round-trip external cavity loss mainly comes from the three aspects: etalon’s peak insertion loss, grating loss and loss caused by limited MEMS size. The first term is related to the etalon manufacturing and usually less than 1dB in our measurement. The second term is mainly determined by the diffraction efficiency of the grating (90% for the given grating), and is about 1dB. So the total external cavity loss will be less than 5dB even by considering the imperfect AR coating loss of every optical surface.

3.2 Simulation of the laser’s characteristics

For an ECTL used in coherent system, high wavelength accuracy and narrow spectral linewidth are expected. In this part, based on the calculated TOF bandwidth and total external cavity loss, improved TDTW method [19] is used to simulate the characteristics of the MEMS ECTL. The passive parts are firstly modeled in frequency domain based on the calculated TOF bandwidth and total external cavity loss in section 3.1, while gain chip is simulated in the conventional time domain. The calculated spectrum of the external cavity is then transformed to the time domain via digital filters.

In [10], the authors studied the impact of the etalon’s reflectivity and external cavity loss on the wavelength accuracy of the ECTL. However, the wavelength locking should consider both wavelength accuracy and a reliable operation, for mode-hop is a common phenomenon in ECTL. Figure 4(a) and Fig. 4(b) show the frequency offset to the etalon’s transmission peak and the continuous tuning range with different etalon reflectivity and the external cavity loss when the transmission peaks of the etalon and the TOF are perfectly aligned to the ITU-T grid as shown in Fig. 1.

Fig. 4 (a) Wavelength accuracy. (b) Continuous tuning range as a function of etalon’s reflectivity with different external cavity loss.

Download Full Size | PDF

As depicted in Fig. 4(a), the frequency offset to the etalon’s transmission peak decreases with the increase of the etalon’s reflectivity and the decrease of the external cavity loss which agrees with the simulation results in [10]. Thus, increasing the etalon’s reflectivity is an effective way to obtain high wavelength accuracy, and the etalon’s transmission peak should be shifted to match the ITU-T grid. However, from Fig. 4(b), higher reflectivity of the etalon has a negative impact on the continuous tuning range of the ECTL, and a small tuning range will make the wavelength hard to control because the mode-hop occurs during the phase tuning. Also from Fig. 4(a), we find that the lasing wavelength never occurs at the shorter-wavelength side of the etalon’s transmission peak in this condition. This is due to the asymmetric mode pulling [20] which causes an acceleration of the phase accumulation on the shorter-wavelength side and a deceleration of the phase accumulation on the longer-wavelength side. To overcome this problem, we improve the alignment condition between the transmission peaks of the etalon and the TOF as illustrated in Fig. 5 . The TOF’s transmission peak is shifted to make sure the etalon’s transmission peak is at its longer-wavelength side, thus the combined external cavity spectrum will become asymmetric and shift to the shorter-wavelength side.

Fig. 5 Schematic diagram showing the improved alignment scheme between the transmission peaks of the etalon and the TOF to optimize the tuning behavior of the ECTL.

Download Full Size | PDF

In this situation, the asymmetric mode pulling will be compensated. The improvement of tuning characteristics can be clearly seen in Fig. 6 . When we use the peak to peak alignment, the output power of the laser is monotone during the phase tuning with a mode-hop-free tuning range of 4GHz as shown in Fig. 6(a), this indicates the wavelength cannot pass the peak of the etalon, thus a close-loop operation to lock the lasing wavelength to the transmission peak (indicating a maximum of the output power) become unreliable and unstable. When the improved alignment is performed, there will be a local maximum of the output power during the phase tuning and an increase of continuous tuning range as shown in Fig. 6(b). This means that the lasing wavelength can pass the transmission peak, the continuous tuning range also increases by about 2GHz to make the wavelength locking operation work in a more stable region without mode-hop. However, one major drawback of this scheme is that the loss of the external cavity will increase which always leads to a higher threshold.

Fig. 6 Simulated output power variation and frequency tuning behavior during the phase tuning when the transmission peaks of the etalon and the grating are (a) peak-to-peak aligned and (b) not peak-to-peak aligned as shown in Fig. 5. The etalon’s reflectivity is 0.65 in the simulation.

Download Full Size | PDF

3.3 Spectral linewidth

We also investigate the linewidth of the ECTL. The theoretical limit of laser linewidth $Δ ν$ is determined by [21]

Δ ν = \frac{α^{2} β^{2} R_{s p} ξ}{4 π S} + \frac{R_{s p} ξ}{4 π (S + S_{e})}

Where $α$ is the linewidth enhancement factor, $β$ can be calculated from the phase condition for the longitudinal modes of the whole cavity [22]. $R_{s p}$ is the spontaneous emission rate which is written by

R_{s p} = \frac{v_{g}^{2} h ν g (ν) a_{m} S}{2 P_{o u t}}

Where, $S$ and $S_{e}$ are the number of photons in the gain medium and external cavity, respectively. $h$ is the Planck constant. $ν$ is the frequency. $a_{m}$ is the threshold loss of the laser given by

a_{m} = \frac{1}{l} \ln (1 / r_{o u t} r_{e f f}) + a_{i n}

Here, $l$ , $a_{i n}$ are the length and internal loss of gain medium. $r_{o u t}$ , $r_{e f f}$ are the reflection of the output port and the effective reflection of external cavity, respectively. $ξ$ is the factor of the external cavity length which can be written as

ξ = \frac{l / v_{g}}{l / v_{g} + l_{e f f} / v_{g c}}

Where $l_{e f f}$ is the effective length of the external cavity which means the total optical path of the light in the laser cavity. Thus it has a fixed relationship with external cavity mode spacing $Δ f_{c a v i t y}$

l_{e f f} = \frac{c}{2 \cdot Δ f_{c a v i t y}}

Where c is the speed of light in vacuum. The calculated linewidth as a function of etalon reflectivity under different cavity loss for the given ECTL is shown in Fig. 7 .

Fig. 7 Simulated spectral linewidth of the ECTL as a function of etalon’s reflectivity for different external cavity loss.

Download Full Size | PDF

From Eq. (3) to Eq. (6) as well as Fig. 7, by improving $r_{e f f}$ to reduce the total cavity loss and increasing $l_{e f f}$ , we can get a narrower linewidth, which has been experimentally evaluated in [18, 23–25]. The theoretical linewidth is less than 30 kHz even when the total cavity loss is 6dB as shown in Fig. 7. What’s more, for a given cavity length, $l_{e f f}$ can be extended by increasing the etalon’s reflectivity [26] as it is almost directly proportional to $1 / (1 - r_{F P})$ . However, a high etalon’s reflectivity will result in a narrow tuning range which makes the wavelength locking operation unstable.

According to the analysis in section 3.2 and 3.3, the etalon’s reflectivity has impacts on wavelength accuracy, continuous tuning range and linewidth, so it is important to propose an optimal value of etalon’s reflectivity. From Fig. 4(a), it is found that if the etalon’s reflectivity is larger than 0.6, the frequency offset to ITU-T grid will be less than about 2.5GHz (upper limit of frequency offset in 50GHz DWDM grid) even when the cavity loss is 6dB. Moreover, although the frequency offset to ITU-T grid decreases (indicating higher wavelength accuracy) with the increase of the etalon’s reflectivity, it reaches a floor at the reflectivity of about 0.7 as depicted in Fig. 4(a). While the continuous tuning range still decreases obviously with the increasing of the etalon’s reflectivity in the region, where the etalon’s reflectivity is larger than 0.7 as shown in Fig. 4(b). Thus, from the aspect of reliable wavelength locking operation considering both wavelength accuracy and continuous tuning range, the optimal range of etalon’s reflectivity is from 0.6 to 0.7. As shown in Fig. 7, we found that compared with the increasing of the etalon’s reflectivity, the decrease of the total cavity loss has a more significant impact on linewidth reduction. This suggests that increasing the etalon’s reflectivity is not an effective way to reduce linewidth. Thus, conclude of the optimal value of the etalon’s reflectivity range from 0.6 to 0.7 can be conducted by considering wavelength accuracy, continuous tuning range and linewidth.

4. Fabrication and characterization of the MEMS ECTL module

Figure 8 shows the 3-D layout of the ECTL including MEMS based external cavity and output optics, the etalon’s reflectivity is 0.65 based on the analysis of section 3. The total physical length of the laser cavity is designed to be about 12mm, and the external cavity mode spacing $Δ f_{c a v i t y}$ is 8.5GHz. The output is coupled through a pair of collimating lens and a dual-stage isolator with a high isolation of more than 45dB. The two parts are fixed on a ceramic substrate mounted on a thermoelectric controller (TEC), the single-axis MEMS mirror is packaged in a TO can to maintain the reliability. The whole ECTL is then placed in a 28-pin butterfly module with hermetic package. The module can be integrated with a control circuit compatible with OIF-ITLA-MSA-01.2 as shown in Fig. 8, and all the following measurements are based on the packaged ITLA module.

Fig. 8 Layout of the MEMS ECTL module (the inset is a photograph of the packaged ITLA).

Download Full Size | PDF

We first measured the basic characteristics of the ITLA, Fig. 9(a) is the measured spectrums for 100 channels in C-band (1528.77nm~1568.36nm in vacuum), the ECTL module exhibits high output power of more than 13dBm over the entire C-band when the injection current is 300mA, single-mode operation with high side mode suppression ratio (SMSR) of more than 55dB can also be observed. What’s more, high wavelength accuracy can be obtained using the improved alignment as shown in Fig. 9(b), ± 1GHz frequency deviation to ITU-T channels for over the entire tuning range can be achieved.

Fig. 9 (a) Wavelength tuning characteristics showing 100 channels with 50GHz grid in C-band. (b) Wavelength accuracy of the ITLA module in C-band.

Download Full Size | PDF

The measured relative intensity noise (RIN) is less than −145dB/Hz from 10MHz to 20GHz at all channels, and three typical RIN spectrums in C-band are shown in Fig. 10(a) . The spectral linewidth is measured using delayed self-heterodyne (DSH) method with a 20-km single-mode fiber and a LiNbO₃ intensity modulator to shift the carrier frequency by 300MHz. Thanks to the long external cavity and low-noise DC source, the linewidth of the developed ITLA is less than 50 kHz over the entire tuning range as shown in Fig. 10(b), in most cases, the linewidth is less than 30 kHz which matches well with the calculated results in section 3.3. Notice that the measured linewidth in the central part of the C-band is narrower than that in the two edges of the C-band. This is because the wavelengths in the two edges of the C-band have larger diffraction angles, and the limited MEMS mirror size will lead to an extra wavelength-dependent coupling loss. Thus by reducing the total external cavity loss through optimizing the MEMS mirror design, the round-trip external cavity loss can be reduced by 1.89dB, and the linewidth can be less than 20 kHz as demonstrated in section 3.3.

Fig. 10 (a) Measured RIN results of the ITLA. (b) Spectral linewidth of the ITLA in C-band.

Download Full Size | PDF

It is also worth mentioning that the DSH method gives an overestimation of the linewidth due to the low-frequency flicker noise [27, 28], and the measured linewidth shape exhibits a Voigt shape rather than a Lorentzian shape [28], so the actual linewidth of this device is even narrower, and a phase noise test platform is required for precise linewidth characterization. Normally, the linewidth broadening due to the flicker noise can be ignored in coherent system applications, and we believe this ITLA module can fully meet the linewidth requirement of commercial 100Gbit/s PM-QPSK coherent system and has the potential in the application of higher rate (400Gbit/s and 1Tbit/s) coherent systems with multi-level modulation formats.

5. Experimental verification of the ITLA module

5.1 112Gbit/s PM-QPSK experiment

First, an 112Gbit/s PM-QPSK system is constructed to verify the ITLA’s performance, the experimental setup is shown in Fig. 11(a) .

Fig. 11 (a) 112Gbit/s PM-QPSK experimental setup. (b) BER versus OSNR curves for different ITLA. Inset is the constellation diagram for the proposed ITLA. ASE: amplified spontaneous emission, VOA: variable optical attenuator, OSA: optical spectral analyser.

Download Full Size | PDF

At transmitter side, the output of a commercial ITLA is modulated by a PM-QPSK modulator driven by 2³¹-1 pseudo-random binary sequence (PRBS) data with data rate of 27.95Gbit/s, the resulting signal is thus a PM-QPSK signal with a data rate of 112Gbit/s. The intradyne coherent receiver consists of a polarization-diversity 2 × 8 mixer integrated with four balanced photodiodes. The proposed ITLA and a commercial narrow linewidth ITLA (NLW ITLA) with typical linewidth about 300 kHz are used as LO lasers for comparative study, and the wavelengths of both transmitter laser and LO laser are set to be 1545.32nm. The four demodulated signal components (I_x, I_y, Q_x, Q_y) are then digitalized asynchronously using an 80Gsymbol/s real-time oscilloscope, digitized waveforms of 2-million samples are processed offline in a computer. Traditional demodulation algorithms for single carrier are used in the experiment, such as constant-modulus algorithm (CMA). The measured BER curve as a function of received OSNR is shown in Fig. 11(b), and a slight OSNR improvement can be observed in this back-to-back experiment. Compared with the commercial narrow linewidth ITLA, the system performance can be improved by 0.5dB at 10⁻² BER when using the proposed ITLA.

5.2 12.5Gbit/s multi-carrier CO-OFDM experiment

In CO-OFDM receiver, a FFT is used to convert the time-domain signal to frequency domain for each OFDM symbol. During the period of each OFDM symbol, the phase noise is estimate using pilot subcarrier information. Usually, the phase noise in one OFDM symbol is assumed to be uniform, because the symbol rate is much higher than the linewidth. However, for long symbol length or FFT size, the accuracy will be reduced, since phase noise may vary during each symbol period. Base on this, we also carry out a CO-OFDM experiment to evaluate the proposed ITLA. Figure 12 shows the typical CO-OFDM experimental setup, which has a similar configuration as [16]. The demodulated signals are first detected by four pairs of balanced detectors. The four RF signals corresponding to I/Q components are then fed to an real-time oscilloscope, acquired at 80 Gsymbol/s, and processed off-line using MATLAB program. The BER is derived by comparing the transmitted and recovered data. The length of each data is ~10⁶. We use three different tunable lasers: a conventional ECTL (Agilent N7714) with typical linewidth less than 100 kHz, a commercial NLW ITLA with typical linewidth of 300 kHz and the proposed ITLA for comparative experiment. Four different schemes are tested: (1) number of used subcarrier is 80, and FFT size is 128; (2) number of used subcarrier is 160, and FFT size is 256; (3) number of used subcarrier is 320, and FFT size is 512; and (4) number of used subcarrier is 640, and FFT size is 1024. The experimental results of BER curves are shown in Fig. 13 , we measure more than 10 times for each OSNR point in Fig. 13.

Fig. 12 12.5Gbit/s CO-OFDM experimental setup for the proposed ITLA test. AWG: arbitrary wavefrom generator.

Download Full Size | PDF

Fig. 13 BER versus OSNR curves in CO-OFDM experiment. (a) FFT size is 128. (b) FFT size is 256. (c) FFT size is 512. (d) FFT size is 1024.

Download Full Size | PDF

Figure 13(a) shows that the three tunable lasers have similar BER performances. The implementation of the proposed ITLA can bring a 0.5dB OSNR improvement compared to the commercial ITLA. Better performance can be obtained with the increase of the FFT size by using the proposed ITLA. We can observe a 4dB ONSR improvement at 10⁻³ BER when using the proposed ITLA compared with the commercial ITLA from Fig. 13(b). This improvement becomes more obvious when the FFT sizes are 512 and 1024 as shown in Fig. 13(c) and 13(d). The use of the proposed ITLA also enables an error-free transmission, while, by using the commercial ITLA, we can only realize a BER higher than 10⁻², which has already exceeded the forward error correction (FEC) limit. The conventional ECTL only shows a slightly better performance than the proposed ITLA. This may due to its distinguished mechanical stability as a scientific instrument.

6. Conclusion

A compact, ultra-narrow linewidth MEMS ECTL for coherent detection is presented in this paper. The optimal ECTL design is discussed based on a two-step hybrid analysis method to obtain high wavelength accuracy and narrow linewidth. In order to make the wavelength locking operation stable and reliable, the transmission peaks of both the etalon and the TOF should be pre-shifted to compensate the asymmetric mode pulling effect. The etalon’s reflectivity should be chosen by considering the wavelength accuracy and continuous tuning range as well as the linewidth of the ECTL. The device can be placed in a 28-pin butterfly module compatible with OIF-MSA-ITLA-01.2, a wide wavelength tuning range about 40nm in C-band with a narrow linewidth of less than 50 kHz and a high wavelength accuracy of ± 1GHz over the entire tuning range can be obtained. Future work shall focus on the optimization of the MEMS mirror design to further reduce the linewidth.

The performance of the ECTL has also been evaluated in single-carrier PM-QPSK and multi-carrier CO-OFDM systems. This device shows a slightly-improved performance in 112Gbit/s PM-QPSK system compared with commercial NLW ITLA, and reveals an excellent performance in CO-OFDM systems especially when the FFT size is larger than 256. This ECTL can fully meet the requirement of commercial PM-QPSK coherent system and has the potential in the application of higher rate (400Gbit/s and 1Tbit/s) coherent systems with higher-order modulation formats.

Acknowledgments

The authors gratefully acknowledge Dr. Zhang Zhang in Fiberhome Technologies Co., Ltd. for PM-QPSK experimental setup, Liu Yang and Bo Zhang in Accelink Technologies Co. Ltd. for helpful discussions. This work is supported in part by the National High Technology Research and Development Program(“863”Program) of China (Grant No. 2008AA01Z207, 2011AA03A106 and 2011AA010304).

References and links

1. T. J. Xia, G. A. Wellbrock, Y. K. Huang, M. F. Huang, E. Ip, P. N. Ji, D. Y. Qian, A. Tanaka, Y. Shao, T. Wang, Y. Aono, and T. Tajima, “21.7Tb/s field trial with 22 DP-8QAM/QPSK optical supperchannels over 1503-km of installed SSMF,” in Proceeding of Optical Fiber Communication Conference (Los Angeles, USA, 2012), paper PDP5D.6.

2. O. Interconn. Forum, “Implementation agreement for integrated polarization multiplexed quadrature modulated transmitters,” in Proc. Opt. Internetw. Forum, (Mar. 2010), pp. 1–20. www.oiforum.com

3. S. L. Zhang, M. F. Huang, F. Yaman, E. Maeto, D. Y. Qian, Y. Q. Zhang, L. Xu, Y. Shao, I. Djordjevic, T. Wang, Y. Inada, T. Inoue, T. Ogata, and Y. Aoki, “40×117.6Gb/s PDM-16-QAM OFDM transmission over 10181 km with soft-decision LDPC coding and nonlinearity compensation,” in Proceeding of Optical Fiber Communication Conference (Los Angeles, USA, 2012), paper PDP5C.4.

4. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express 17(11), 9421–9427 (2009). [CrossRef] [PubMed]

5. R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s Continuous Waveband PDM‐OFDM‐FDM Signal with Spectral Efficiency of 3.3 bit/s/Hz over 400 km of SSMF,” in Proceeding of Optical Fiber Communication Conference (San Diego, USA, 2012), paper PDP C2.

6. S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser Linewidth Tolerance of Decision-Aided Maximum Likelihood Phase Estimation in Coherent Optical M-ary PSK and QAM Systems,” IEEE Photon. Technol. Lett. 21(15), 1075–1077 (2009). [CrossRef]

7. M. Seimetz, High-Order Modulation for Optical Fiber Transmission (Springer Series in Optical Sciences, 2009), Chap. 7.

8. I. Fatadin and S. J. Savory, “Impact of phase to amplitude noise conversion in coherent optical systems with digital dispersion compensation,” Opt. Express 18(15), 16273–16278 (2010). [CrossRef] [PubMed]

9. C. Xie, “Local Oscillator Phase noise Induced Penalties in Optical Coherent Detection Systems Using Electronic Chromatics Dispersion Compensation,” in Proceeding of Optical Fiber Communication Conference (San Diego, USA, 2012), Paper OMT4.

10. J. De Merlier, K. Mizutani, S. Sudo, K. Sato, and K. Kudo, “Wavelength channel accuracy of an external cavity wavelength tunable laser with intracavity wavelength reference etalon,” J. Lightwave Technol. 24(8), 3202–3209 (2006). [CrossRef]

11. K. Sato, K. Mizutani, S. Sudo, K. Tsuruoka, K. Naniwae, and K. Kudo, “Wideband external cavity wavelength-tunable laser utilizing a liquid-crystal-based mirror and an intracavity etalon,” J. Lightwave Technol. 25(8), 2226–2232 (2007). [CrossRef]

12. A. Q. Liu and X. M. Zhang, “A review of MEMS external-cavity tunable lasers,” J. Micromech. Microeng. 17(1), R1–R13 (2007). [CrossRef]

13. E. Ip, J. M. Kahn, D. Anthon, and J. Hutchins, “Linewidth measurements of MEMS-based tunable lasers for phase-locking applications,” IEEE Photon. Technol. Lett. 17(10), 2029–2031 (2005). [CrossRef]

14. X. M. Zhang, A. Q. Liu, C. Lu, and D. Y. Tang, “Continuous wavelength tuning in micromachined Littrow external-cavity lasers,” IEEE J. Quantum Electron. 41(2), 187–197 (2005). [CrossRef]

15. W. R. Trutna Jr and L. F. Stokes, “Continuously tuned external cavity semiconductor laser,” J. Lightwave Technol. 11(8), 1279–1286 (1993). [CrossRef]

16. N. Kaneda, A. Leven, and Y.-K. Chen, “Block length effect on 5.0 Gbit/s real-time QPSK intradyne receivers with standard DFB lasers,” Electron. Lett. 43(20), 1106–1107 (2007). [CrossRef]

17. X. Yi, W. Shieh, and Y. Ma, “Phase noise effects on high spectral efficiency coherent optical OFDM transmission,” J. Lightwave Technol. 26(10), 1309–1316 (2008). [CrossRef]

18. S. D. Saliba and R. E. Scholten, “Linewidths below 100 kHz with external cavity diode lasers,” Appl. Opt. 48(36), 6961–6966 (2009). [CrossRef] [PubMed]

19. W. Li, W. P. Huang, and X. Li, “Digital filter approach for simulation of a complex integrated laser diode based on the traveling wave model,” IEEE J. Quantum Electron. 40(5), 473–480 (2004). [CrossRef]

20. S. L. Sochava and W. B. Chapman, “Intra-cavity etalon with asymmetric power transfer function,” US Patent, 7061946 B2, 1–18 (2006).

21. H. Stao and J. Ohya, “Theory of spectral linewidth of external cavity semiconductor lasers,” IEEE J. Quantum Electron. 22(7), 1060–1063 (1986). [CrossRef]

22. T. Fujita, J. Ohya, S. Ishizuka, K. Fujito, and H. Sato, “Oscillation frequency shift suppression of semiconductor lasers coupled to external cavity,” Electron. Lett. 20(10), 416–417 (1984). [CrossRef]

23. O. Nilsson, S. Saito, and Y. Yamamoto, “Oscillation frequency, linewidth reduction and frequency modulation characteristics for a diode laser with external grating feedback,” Electron. Lett. 17(17), 589–591 (1981). [CrossRef]

24. N. Olsson and J. Van Der Ziel, “Performance characteristics of 1.5-µm external cavity semiconductor lasers for coherent optical communication,” J. Lightwave Technol. 5(4), 510–515 (1987). [CrossRef]

25. H. Loh, Y. J. Lin, I. Teper, M. Cetina, J. Simon, J. K. Thompson, and V. Vuletić, “Influence of grating parameters on the linewidths of external-cavity diode lasers,” Appl. Opt. 45(36), 9191–9197 (2006). [CrossRef] [PubMed]

26. A. Romano, M. D. Donno, and A. Pianciola, “Phase-control in an external-cavity tunable laser,” US Patent, 7505490 B2, 1–24 (2009).

27. S. Camatel and V. Ferrero, “Narrow linewidth CW laser phase noise characterization methods for coherent transmission system application,” J. Lightwave Technol. 26(17), 3048–3055 (2008). [CrossRef]

28. N. H. Zhu, J. W. Man, H. G. Zhang, J. H. Ke, W. Han, W. Chen, Y. Liu, X. Wang, H. Q. Yuan, and L. Xie, “Lineshape analysis of the beat signal between optical carrier and delayed sidebands,” IEEE J. Quantum Electron. 46(3), 347–353 (2010). [CrossRef]

Compact MEMS external cavity tunable laser with ultra-narrow linewidth for coherent detection

Abstract

1. Introduction

2. Configuration of the MEMS ECTL

3. Simulation and design of the MEMS ECTL

3.1 Simulation of the passive external cavity

3.2 Simulation of the laser’s characteristics

3.3 Spectral linewidth

4. Fabrication and characterization of the MEMS ECTL module

5. Experimental verification of the ITLA module

5.1 112Gbit/s PM-QPSK experiment

5.2 12.5Gbit/s multi-carrier CO-OFDM experiment

6. Conclusion

Acknowledgments

References and links

Cited By

Figures (13)

Equations (7)

Optics Express