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A novel and cost-efficient technique is presented to generate non-return-to-zero (NRZ) and ultra-wideband (UWB) signals in different time slots of time division multiplexing-passive optical network (TDM-PON) by using a single chirped controlled semiconductor laser associated with an optical bandpass filter. In this technique, the chirp of the laser is controlled by different bias burst amplitudes (BBA) for different time slots. Through the proper selection of the burst amplitudes, 10 Gbps NRZ and 1.25 Gbps UWB signals are generated in different time slots. Principle of operation is discussed, the complete chirp behavior of the laser is experimentally investigated, data transmission of the generated signals is demonstrated and bit-error-rate (BER) level of 10−9 is achieved.
© 2016 Optical Society of America
1. Introduction
Low complexity, cost and power consumption and high data rate are the most important issues to be considered when developing the current wireless local-area network (WLAN) and Wireless personal-area network (WPAN). Ultra-wideband (UWB) is a promising solution to fulfill these demands due to its low power consumption, immunity to multipath fading, interference mitigation, carrier free, high data rate and capability to penetrate through obstacles [1]. The unlicensed use of the UWB spectrum from 3.1 to 10.6 GHz, with a power spectral density (PSD) lower than -41.3 dBm/MHz is regulated by U.S. Federal Communications Commission (FCC) [2]. Due to the low PSD, the communication distance of UWB is limited to a few meters. To extend the coverage area, UWB over fiber (UWBoF) is introduced [3]. To make the UWBoF systems practical for the commercial deployment, it is necessary to simplify the operation, ease the maintenance and reduce the general cost. One of the simplest and most cost efficient techniques has been proposed and experimentally demonstrated by the authors [4–6]. The aforementioned approach is based on the direct modulation of a semiconductor laser (DML), optical filtering and chromatic dispersion in a transmission fiber. When an electrical pulse is applied to the distributed feedback (DFB) laser, not only the intensity, but also the optical frequency of the output light is modulated (Fig. 1(a)). The modulated output intensity is related to the photon density inside the cavity and the laser frequency modulation is due to the carrier density change. Due to the carrier effect there is a time delay (τ) between intensity modulation (IM) and frequency modulation (FM) (Fig. 1(a)), which was observed to be between 15 and 25 ps [4]. Conversion of FM to IM can be achieved, by positioning the spectrum of the laser output on the negative slope of the optical bandpass filter (OBPF) (Fig. 1(b)), so that IM and FM are combined and negative first derivative Gaussian pulse (monocycle) is generated at the output (Fig. 1(c)). The DFB and OBPF are commercially available in a so-called chirped-managed-laser (CML) package.
Fig. 1 (a) Optical IM and FM generated by a DML; (b) optical FM-to-IM conversion by optical filtering; (c) optical output waveform obtained by combination of IM and FM-to-IM conversion. Δf: frequency deviation, 15 ps < τ < 25 ps (Ref. [4], Fig. 1).
On the other hand, CML is an alternative transmitter technology that allows a directly modulated laser (DML) to be used in high-performance optical transmitters. Consider a “1 0 1” bit sequence from a DML. As this signal travels through the fiber, dispersion spreads the energy of the 1 bits into adjacent 0 bits and would normally close the eye diagram. The CML consists of a DFB laser and one OBPF and takes advantage of the chirp of the DML to solve this problem. This approach is based on the continuous phase shift of the optical carrier associated with the adiabatic chirp of the laser together with the action of a passive optical filter for spectral reshaping. For the same “1 0 1” bit sequence in CML the 1 bits have higher optical frequency than the 0 bits by 1/2 of the bit rate . This causes the phase of the output light to slip by p, making the second 1 bit p out of phase with the first 1 bit. This phase shift is the key to maintaining a clean 0 bit. The energy of the 1 bits still spreads into the middle 0 but due to the destructive interference, the eye remains open after more than 200 km of transmission [7].
Fixed fiber deployments for fiber to the home (FTTH) services are increasing based on PON technology. Gigabit PON and Ethernet PON, both based on time-division multiplexing (TDM), provide services to N users by use of passive 1:N power splitters with an aggregate bit rate [ITU-T G.984.2]. In the downstream direction, the optical line terminal (OLT) dedicates time slots to the multiple subscribers (Fig. 2). The optical network units (ONUs) and Radio antenna units (RAU) receive their own data through the address labels embedded in the signal. To reduce the overall cost of the system, it is desirable to investigate the compatibility of the proposed system in [4–6] with the TDM-PON.
As discussed above, CML can generate both NRZ and UWB signals. Yet, to generate the UWB signal, the signal spectrum should be placed on the filter’s negative slope [4], while in case of NRZ generation, the signal spectrum should be placed on the filter’s positive slope [7].
For the first time, to the best of our knowledge, we propose a technique to generate the NRZ and UWB signals from a single chirped-managed-laser (CML) module consisting of a semiconductor laser and an optical bandpass filter in TDM-PON system to reduce the cost and complexity of the transmitter. In this approach the laser operation point in each time slot of the TDM structure is controlled through the different bias burst amplitudes (BBA) at different time slots. In this way, the chirp of the laser is controlled and the spectrum of the optical signal is placed in the required position according to the central wavelength of the optical filter, to generate either NRZ or UWB signal.
In Section 2, the chirp behavior of the used CML is measured and analyzed. In Section 3, NRZ and UWB are first generated separately in burst mode, to study the chirp effects in burst operation mode. Then, both signals are generated in different time slots of a burst signal by a single light source. The results are finally summarized in Section 4.
2. Chirp analysis of CML
The chirp of the DFB laser plays the main role in this work, therefore it would be beneficial to study its behavior. Three main mechanisms are involved in changing the optical frequency in a semiconductor laser: 1) transient chirp, associated with the relaxation oscillation; 2) adiabatic chirp, introduced by the effect of the injection current on the refractive index of the cavity and 3) thermal chirp, where the temperature affects both the refractive index and cavity length [8]. In the low frequency region, the frequency deviation is governed by thermal effects. It is almost independent of bias current but a large frequency deviation is caused by temperature change, which is caused by the modulation current [9]. As it is theoretically explained and experimentally shown in [9], the frequency deviation is proportional to the temperature modulation in an active layer negatively (−ΔT(t)) and subsequently to −i(t). Therefore, it is expected that the FM and IM be out of phase in low frequencies. At frequencies higher than 10 MHz, the frequency chirp is caused by the carrier effect and can be expressed as:
where P(t) and ϕ(t) are the instantaneous optical power and phase, respectively. α is the linewidth enhance factor and κ is the adiabatic chirp coefficient. The first term in Eq. 1 denotes the adiabatic chirp. The second term presents the transient chirp and can be neglected when the laser is biased far from the threshold. Therefore, Eq. 1 can be simplified as:As P(t) is proportional to the laser driving current I(t), the frequency deviation is also directly proportional to I(t). The chirp variation of the DFB laser versus modulation amplitude is reported in [6,9].
In this work, the laser chirp in different modulation frequency regions is used to establish a TDM-PON compatible transmitter to generate UWB and NRZ signals. The adiabatic chirp in the high frequency region is used to generate the UWB pulses as reported in [4]. In the low frequency region, the thermal chirp is used to build the TDM burst mode. In our previous publications [4, 6], the laser was directly modulated at 10 GHz in continuous-mode. In the this work, the laser will be modulated at a low frequency, depending on the frequency of the burst signal. Therefore, a chirp measurement is required to find a frequency with the sufficient chirp in a low frequency region. The central wavelength, input impedance and threshold current of the laser module are 1538 nm, 50 Ω and 15 mA, respectively. The laser bias current is set to 40 mA. The integrated OBPF is a multiple cavity filter with 3-dB bandwidth of 0.06 nm and 10-dB bandwidth of 0.12 nm. The laser is driven by a sinusoidal electrical signal at different frequencies. For each frequency the filter of the CML is tuned by temperature regulation, so that maximum eye diagram opening is achieved at the CML output. The chirp and the phase difference between FM and IM are measured by using the time-resolved chirp-measurement technique [10] and the results are depicted in Fig. 3.
A 5 GHz/mA laser thermal chirp at frequencies around 1 KHz can be seen. The phase difference between FM and IM in low frequency is close to 180°, which confirms the negative sign in the theoretical discussion above on the relation between the thermal chirp and the modulation current. The laser chirp in the high frequency region is about 5 GHz/mA due to adiabatic effect, as was expected for a 10 Gbps CML [7]. The phase difference in this region is close to 30°, which refers to the delay (τ) between IM and FM in Fig. 1. In the middle region, the thermal and adiabatic effects cancel each other out, leading to a decrease in chirp. Based on the theoretical analysis and experimental measurement results, it is expected that the chirp follows the intensity at high frequencies with a time delay τ and in low frequency region to be 180° out of phase.
As mentioned in Sec. 1, to generate the NRZ and UWB signals, the signal’s spectrum should be placed on positive and negative filter slopes, respectively. Therefore, the burst frequency is chosen as 1 KHz, so that the chirp at this frequency is sufficient to move the signal spectrum between the positive and negative slops of the OBPF.
The main idea of this work is to generate different signal forms (NRZ and UWB) by changing the BBA without moving the OBPF central wavelength. Before moving to the next section we test this idea in the continuous-mode. The laser bias current, laser temperature and filter temperature are set to 25 mA, 20°C and 41°C, respectively. First, an electrical NRZ signal with 0 mA dc-offset and modulation current of ipp = 20 mA (vpp = 1 V) is sent to the laser. In order to locate the optical spectrum of the laser on proper position with respect to the position of the filter, the dc-offset is increased to change the laser operation point and consequently the frequency deviation. Best optical eye diagram was observed for Idc−off set = 14.6 mA (Vdc−off set = 0.73 V). The experiment is repeated for UWB signal and Idc−off set = 34 mA (Vdc−off set = 1.7 V) was recorded at the point of the best quality of the eye diagram. These levels help us to find the suitable intensity of the BBA in each time slot. This test was performed in continues-mode and high frequency, where adiabatic chirp dominates. The frequency deviation in this region is proportional to the laser operation point (Eq. 2). Therefore, a lower laser operation point was reported to place the signal spectrum on the positive slope of the filter (lower frequency) to generate the NRZ signal; a higher laser operation point was reported to place the spectrum on the negative one (higher frequency) to generate the UWB signal.
In the burst mode, the laser works at low frequency (1 KHz) and regarding the theoretical discussion and Fig. 3, the higher laser operation point in this region leads to a lower thermal chirp and vice versa. This is shown schematically in Fig. 4. The NRZ and UWB signals are added to the time slots with the higher and lower intensity levels, respectively. From the other side, burst frequency is chosen around 1 KHz and therefore, the higher the driving current the lower the thermal chirp. Consequently the spectrum of the NRZ and UWB signals would be at lower frequency (positive filter slope) and higher frequency (negative filter slope), respectively.
3. Experiment
3.1. Burst mode
In this subsection, 1.25 Gbps UWB and 10 Gbps NRZ signals are separately generated in the burst mode to study the effects of thermal chirp on the signal quality. Figure 5 shows the transmitter of the setup. A 10 Gbps pulse pattern generator (PPG) is programmed in the alternate (ALTN) mode. ALTN-A contains a 1.25 Gbps coded electrical on-off-keying signal (PRBS 27 − 1) generated by PPG at 10 Gbps and ALTN-B is all zero. From the coding, a logical ‘1’ is represented by “1000 0000” (one ‘1’ bit every 8 bits), and a logical ‘0’ is represented by “0000 0000”. Burst signal is generated from an arbitrary waveform generator (AWG) with an intensity of ipp = 20 mA (vpp = 1 V) at 1 KHz (Fig. 6). The duration of the high level time slot is 0.45 ms. The PPG and AWG are synchronized; when PPG is in ALTN-A mode, the burst signal has a high level and when PPG is in ALTN-B mode, the amplitude of the burst signal is low. The data and burst signals are then added together through an electrical coupler. To achieve the Gaussian shaped pulses, the generated electrical pulses are curved by an electrical low pass filter (LPF) with a bandwidth of 7.46 GHz and sent to the laser. The laser bias current and laser temperature are set to 20 mA, 20°C, respectively. The filter temperature is tuned to 33.4°C, to generate the monocycle. Figure 7(a) shows the generated optical signal at the output of the transmitter. This experiment is repeated but this time ALTN-A is a 10 Gbps NRZ (PRBS 27 − 1). To have the largest open eye diagram, filter temperature is set to 38.5°C. The optical NRZ signal at the output of the transmitter is depicted in Fig. 7(b).
Two different receivers are implemented to receive the NRZ and UWB signals separately (Fig. 8). The optical signal in case of NRZ is received by an avalanche photodiode (APD) and converted to the electrical signal (Fig. 9(a)). The bit error rate tester (BERT) works in the burst mode, in order to receive only the data and be off in the zero time slots. An electrical signal is generated utilizing a second AWG at 1 KHz to gate the BERT (Fig. 9(c)); so the bit-error-rate (BER) is only measured, when the gating signal has the high level.
The UWB receiver is illustrated in Fig. 8(b). The received signal after APD is shown in Fig. 9(b). Wireless transmission is performed utilizing a pair of UWB planar spiral antenna with a cut off frequency at 2 GHz. A local oscillator is used to down-convert the signal. After amplification, a low-pass filter with a 3-dB bandwidth of 1 GHz is used to remove residual high frequency components. The electrical signal used to gate the BERT is the same as Fig. 9(c). It can be seen from Fig. 9(a) and 9(b) that the slow thermal chirp causes an undesired long rise time, which leads to the degradation of eye diagram. To have an open eye diagram, some portion of the data at the beginning of each time slot must be omitted. Therefore, the duration of the high level time slot of the gate control signal is chosen as 0.4 ms, while the duration of the high level time slot of the burst signal in Fig. 6 is 0.45 ms. The black dashed lines in Figs. 9(a) and 9(b) show how the received data is gated. BER and the eye diagram of the received signals are depicted in Fig. 10.
It is noteworthy to mention that it is also possible to speed up the burst response by operating the laser at higher burst frequencies. The key point is the chirp of the laser, which should be enough to shift the spectrum from one slope of the filter to the other one. As discussed, the amount of the required chirp is about 5 GHz. For this work we chose 1 KHz as the burst frequency and therefore the thermal chirp is dominant which makes the burst response slow. By choosing a burst frequency higher than 10 MHz which causes also a sufficient amount of chirp (e.g. 50 MHz in Fig. 3), the adiabatic chirp will be dominant, leading to a fast burst response. However we have to take into account that these frequencies are higher than 10 MHz and as explained (Eq. 2), in this region the chirp is directly proportional to the applying current; therefore BBAs should be chosen carefully. For instance, in Fig. 4, the BBA of the NRZ is higher than the BBA of the UWB to place the spectrum on the right position, since burst frequency is 1 KHz. But if we choose 50 MHz as the burst frequency, it should be vice versa.
As demonstrated, NRZ and UWB can be generated in burst mode. NRZ and UWB sequences were added to the burst signal and applied to the laser. The filter position was adjusted in each case by thermal regulation to have the largest eye opening. In the receiver, some portion of the data was gated out to achieve an open eye diagram. These techniques can be considered for PON upstream scenario. In the downstream scenario, both signals must be generated in the same burst signal. In this case, the filter position cannot be shifted for each time slot.
3.2. Downstream PON
As mentioned in Subsec. 3.1 it is not possible to move the filter position in different time slots of a burst signal. Instead, we propose to keep the central wavelength of the OBPF fixed and control the position of the signal spectrum. This can be done by controlling the BBA and consequently, the chirp of the laser. The transmitter setup is the same as Fig. 5. ALTN-A contains a 10 Gbps NRZ (PRBS 27 − 1) and ALTN-B is a 1.25 Gbps coded electrical on-off-keying signal (PRBS 27 − 1) generated by PPG at 10 Gbps. Burst signal is generated from AWG with two different intensity levels for NRZ and UWB signals at 1 KHz. When PPG generates NRZ, the burst signal has an amplitude of ipp = 34 mA (vpp = 1.7 V) and when PPG generates the coded on-off-keying, the amplitude of the burst signal is ipp = 14.6 mA (vpp = 0.73 V). The data and burst signals are then added through an electrical coupler (Fig. 11(a)). To achieve the Gaussian shaped pulses, the generated electrical pulses are curved by an electrical low pass filter (LPF) with a bandwidth of 7.46 GHz and sent to the laser. The laser bias current, laser temperature and filter temperature are set to 20 mA, 20°C and 41°C, respectively. Since the bursts have different intensities, the frequency deviation of the optical signal is different in each time slot. Consequently, in each time slot, the optical spectrum is placed at different positions regarding the central wavelength of the OBPF. Figure 11(b) shows the generated optical signal at the output of the transmitter. As it is depicted, optical NRZ and UWB signals are generated by using only a single light source.
Receivers are also the same as in Fig. 8. Electrical signal after APD in both cases is the same as Fig. 11(c). The gating signal to control the burst mode of BERT and receive only one signal format at a time is the same as in Fig. 9(c). In the case of NRZ, the gating signal is adjusted as shown by the dotted-line in Fig. 11(c), so received NRZ data is sent to BERT and UWB data is cut out. In the UWB receiver, the gating signal is set as shown by the dashed-line in Fig. 11(c), therefore, the received UWB data is passed and NRZ data is gated out. BER and eye diagram of the received NRZ and UWB signals are depicted in Figs. 12(a) and 12(b), respectively. As it can be seen, an open eye and BER of 10−9 is achieved at power of −14 dBm and −12 dBm for NRZ and UWB transmission, respectively.
4. Conclusion
A novel technique was presented to generate the NRZ and UWB signals with the use of only one single light source at different time slots of TDM. The proposed approach is based on the direct modulation of a semiconductor laser, optical filtering and TDM architecture. The chirp of the laser was analyzed and principles of operation were explained. 10 Gbps NRZ and 1.25 Gbps UWB signals were generated first separately in burst mode to study the influence of the thermal chirp effects on the signal quality at low frequencies. BER measurements were performed and BER level of 10−9 was achieved. The proposed technique is very cost-efficient, compatible with the TDM-PON architecture and reduces the complexity of the system in the transmitter. The proposed technique is a promising candidate for the future optical access networks specially in fiber-to-the-x scenarios.
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