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We report greatly enhanced transmission distance of multimode vertical-cavity surface-emitting lasers (MM-VCSELs) over standard single-mode fiber using optical injection locking. Transmission distance as high as 90 km is achieved at 10 Gb/s by frequency chirp inversion and higher order transverse mode suppression.
©2010 Optical Society of America
1. Introduction
Directly modulated, multi-transverse mode vertical cavity surface emitting lasers (MM VCSELs) are extensively used in local and storage area networks due to their low cost and wide bandwidth [1]. However, modal and chromatic dispersion prevent them from being used for longer reach networks. Frequency chirp reduction and spectral narrowing must be realized for MM VCSELs to achieve longer distance transmission over standard single mode fiber (SSMF) [2]. Previously, it has been demonstrated that optical injection locking (OIL) of a multi-transverse mode VCSEL can effectively convert it into a single transverse mode device thus mitigating modal dispersion [3]. Moreover, frequency chirp reduction and inversion resulting in transmission distance enhancement has been observed for OIL singlemode VCSELs [4].
In this paper, we present adjustable chirp in a multimode VCSEL using optical injection locking. By adjusting the frequency chirp polarity using OIL, the chirp of the VCSEL can be changed from positive to negative frequency chirp. This negative chirp compensates for chromatic dispersion thereby extending transmission distance. We demonstrate a transmission distance enhancement for a 1550 nm 10 μm aperture multimode VCSEL from 1 km to 90 km at 10 Gb/s. Similarly, for larger aperture (15 μm) devices the transmission distance can be extended from 2 km to 32 km by chirp reduction using OIL. Moreover as an all-optical technique, OIL can be applied to various modulation formats, bit rates, or fiber types and can be applied post-deployment to VCSEL-based transmission systems.
2. Background
Frequency chirp of a VCSEL is determined by the change in carrier density with respect to current modulation. When the carrier density is increased or decreased a frequency transient (transient chirp) occurs after which the frequency of the laser settles to a new shifted value (adiabatic chirp) [5]. For an increasing drive current, the VCSEL carrier density also increases, leading to an increase in the gain. From the Kramers-Kronig relation, this increase in the gain corresponds to a decrease in the real part of the refractive index, which increases the laser frequency. So, a positive transient chirp is observed on the rising edge of an optical pulse, while negative transient chirp is seen on the falling edge. This positive transient chirp, and to a lesser degree the adiabatic chirp, results in pulse spreading due to the positive dispersion of SSMF, where higher frequency components of the pulse travel faster than lower frequency components. Inter-symbol interference (ISI) then results once the pulse has spread beyond one bit period. Optical injection locking has previously been shown to reduce the adiabatic chirp as the frequency of the VCSEL is locked to that of the master laser [6,7]. More recently, we reported an inversion of transient chirp for singlemode OIL VCSELs modulated at 10 Gbps, and the resulting extension of SSMF transmission distance from 10 km to 120 km with a 4 dB power penalty [3]. In the following, we demonstrate the greatly enhanced transmission distance of a directly-modulated multimode VCSEL by combining the spectral narrowing and the data inversion phenomena in optical injection locking.
3. Experimental Setup
Figure 1 shows the experimental setup. The multimode 1550 nm buried-tunnel-junction (BTJ) VCSEL is directly modulated at 10 Gb/s by a pulse pattern generator (PPG) with its output coupled to a single mode lensed fiber [8]. The master laser is a high power distributed feedback (DFB) laser, which is coupled to port 1 of a circulator and then injects into the VCSEL located at port 2. To maximize injection ratio (R = PInj./PVCSEL), a polarization controller is inserted between port 2 and the lensed fiber to match the polarization of the master laser to the VCSEL. The circulator can be replaced with a 3-dB power splitter to lower the cost with a tradeoff in higher master laser output power and negligible performance degradation [9]. From the output of the circulator the data is transmitted over various lengths of SSMF after which it goes to a bit error rate tester, optical spectrum analyzer (OSA), and oscilloscope. Additionally, the output of the circulator is split to a chirpform analyzer, which measures the chirp of the laser in comparison to the modulation pattern.
4. Results
Figure 2 shows the optical spectra of a typical MM VCSEL under 10 Gb/s modulation. Multiple modes spanning 2.5 nm can be seen for the free running case contributing to modal and chromatic dispersion. Under optical injection locking the number of modes is reduced to one and the spectra is narrowed to under 0.3 nm, effectively making the MM VCSEL a single-mode transmitter.
Fig. 2 Optical spectra of the same 15 μm MM VCSEL modulated at 10 Gb/s free-running (top) and under injection locking (bottom). VCSEL bias is shown for each.
Figure 3 shows a 10 Gb/s modulated bit pattern and chirp waveform for a free running 10 μm aperture multimode BTJ VCSEL with a threshold of 4.6 mA and maximum output power of ~5 mW biased at 27 mA. Biased at 24 mA with a 211-1 PRBS at 1.1 Vp-p, the VCSEL has a transient chirp of 12 GHz, adiabatic chirp of 5 GHz, and extinction ratio of 4.9 dB. To improve performance, the VCSEL was then injection locked by a high power CW distributed feedback (DFB) master laser with maximum output power of 80 mW at 375 mA. With a 6 dB injection ratio, VCSEL now biased at 10 mA, the transient chirp is reduced to 2 GHz and adiabatic chirp to 0.5 GHz, with the extinction ratio of 2 dB. However, the bit pattern is now inverted with respect to the drive modulation making the transient chirp negative.
Fig. 3 Measured intensity and chirp waveforms for free-running and R = 6 dB optically injection-locked 10 μm aperture MM VCSELs. The peak-to-peak transient (above line) and adiabatic (below line) chirp are reduced by a factor of 6 through injection locking.
Chirp measurements were also performed for a free running 15 μm aperture multimode BTJ VCSEL with a threshold of 10 mA and maximum output power of 6 mW at 35 mA bias. We modulated the MM VCSEL at 10 Gb/s and injection locked at two different injection ratios (Fig. 4 ). The free running VCSEL biased at 12.2 mA has a transient chirp of 11 GHz and adiabatic chirp of 2 GHz with 4.2 dB extinction ratio. With 3 dB injection ratio the transient and adiabatic chirp is reduced to 7 GHz and 1 GHz, respectively, while extinction ratio remains constant at 4.2 dB. At 6 dB injection ratio the transient chirp is reduced further to 4 GHz and adiabatic chirp to 0.5 GHz, with the extinction ratio reducing to 3.5 dB due to master reflection light. Inversion was not observed for this 15 μm OIL MM VCSEL, due to the limited locking range and poor optical injection efficiency compared to the 10 μm MM VCSEL.
Fig. 4 Measured intensity and chirp waveforms for free-running, R = 3 dB, and R = 6 dB optically injection-locked 15 μm aperture MM VCSELs. The peak-to-peak transient (above line) and adiabatic (below line) chirp are reduced by almost a factor of 3 through injection locking.
Applying this chirp reduction and inversion to a transmission link, extended transmission distance can be achieved by dispersion compensation. Figure 5 shows the power penalty versus SSMF transmission distance for the 10 μm MM VCSEL, where power penalty is referred to the additional required optical power to reach error-free (BER<10−9) as comparing to the free-running VCSEL, 0 km transmission case. For the free-running case the transmission over SSMF is problematic due to large modal and chromatic dispersion resulting in ~1 km of transmission with 4 dB of power penalty. When injection locked with an injection ratio R = 6 dB, the negative chirp causes the bits to compress after propagation over 25 km of SSMF, resulting in a decrease in the needed received optical power to achieve error-free performance or a negative value in power penalty. After longer distance, ~25 km as shown here, the bits begin to disperse again, and hence the power penalty increases. For this case, it takes 90 km fiber distance to reach the same 4-dB power penalty point. This is nearly two orders of magnitude increase in transmission distance compared to the free-running case. Error-free eye diagrams of the OIL VCSEL at the different transmission distances are also shown in Fig. 5.
Fig. 5 Transmission measurements demonstrating distance enhancement of a direct-modulated OIL 10 μm aperture MM VCSEL with negative chirp at 10 Gb/s. Optical eye diagrams for OIL VCSEL at 0 km, after 25-km, 55-km and 85-km transmission are shown.
This transmission distance improvement can also be seen for the 15 μm aperture MM VCSEL (Fig. 6 ). At 10 Gb/s, the free-running VCSEL achieves error-free transmission at −21 dBm received optical power. For the 0 km case, the R = 3 dB OIL case suffers a 1.5 dB power penalty due to the distortions in the pattern, however at 6 dB there is a 2.5 dB improvement in received optical power as the distortion is removed and the overshoot is suppressed. Transmission distance for the free-running MM VCSEL is 2 km with a 4 dB power penalty. The R = 3 dB case can propagate for 8 km before experiencing the same power penalty. At R = 6 dB, transmission distance is extended to ~32 km. This extension is a 16x improvement over the free running MM VCSEL and 2x better than any free-running SM VCSEL or directly modulated DFB laser reported [10].
Fig. 6 Transmission measurements demonstrating distance enhancement of a directly-modulated OIL 15 μm aperture MM VCSEL at R = 3 and 6 dB at 10 Gb/s. Eye diagrams for OIL VCSEL at 0 km and after fiber transmission are shown.
5. Discussion
An explanation of the data pattern inversion phenomenon reported for single mode OIL VCSEL was provided in [4]. We should point out that the same explanation applies to the case reported here, i.e. the directly modulated MM VCSEL under injection locking. However, the model used in ref. 4 was rather incomplete. Particularly, it was not possible to simulate the time-resolved response. Recently, we have developed a more accurate model to explain this phenomenon by realizing that the total output of an OIL VCSEL is indeed a coherent sum (interference) of the reflected master light (by the top distributed Bragg reflector (DBR)) and a locked slave laser [11]. A detailed theoretical treatment and experimental verification of this model is under preparation and will be published elsewhere [12]. A brief discussion is provided here, as the full treatment is beyond of the scope of this paper.
As a VCSEL is injection-locked, the light from the master laser impinges on the top DBR where the majority (>99.5%) of the light is reflected and a small portion (~0.5%) injects into the VCSEL cavity, schematically depicted by Fig. 7 . The light entering the cavity reduces the threshold cavity gain needed to achieve lasing at the master wavelength, which results in an increase of the real part of the refractive index due to the KK relation. This red-shifts the VCSEL transverse modes to longer wavelengths as amplified spontaneous emission peaks. This phenomenon has enabled OIL MM VCSELs to achieve an extended resonance frequency of 54 GHz and 3-dB bandwidth of 38 GHz [1]. In addition, the output light of the VCSEL experiences a phase shift (ΦS) of between –π/2 and cot−1(α), and a change in output power, as dictated by the OIL rate equations [13]. This output then destructively interferes with the reflected master light (with a π phase shift). This phenomenon allows for inverting the optical data pattern of the modulated VCSEL in relation to the applied electrical data pattern [11,13]. Additionally, since the carrier dynamics are governed by OIL rate equations [13], the magnitude of the frequency chirp (adiabatic and transient) can be adjusted by varying the injection ratio of the master laser into the slave measured at the top facet of the VCSEL or the wavelength detuning between master laser and VCSEL. This makes the technique very powerful and, indeed, with great flexibility for optimization.
Fig. 7 Interferometric model of optically injection locked VCSEL. Top path represents light entering VCSEL cavity, where OIL rate equations determine output. Bottom path represents reflection off top facet inducing a ~π phase shift
6. Conclusion
In summary, we have demonstrated extending the transmission distance of a 10 Gb/s MM VCSEL to 90 km over standard single-mode fiber using OIL. By combining two phenomena of OIL-VCSEL, data pattern inversion and chirp reduction, we are able to reduce transient chirp by 6x and adiabatic chirp by 10x while inverting the data pattern with respect to the drive current modulation. OIL of larger aperture VCSELs was also demonstrated with SSMF transmission distance extension of 16 times to 32 km. These results make an optically injection-locked MM VCSEL a strong candidate for future high-speed optical communications.
Acknowledgments
We thank stimulating discussion on the reflection model with W. Yang and G. Peng. This work was supported by DARPA UPR Award HR0011-04-1-0040 and a DoD National Security Science and Engineering Faculty Fellowship via Naval Post Graduate School N00244-09-1-0013.
References and links
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