1.55-μm single-mode VCSEL frequency chirp behavior is investigated in under-threshold and above-threshold operating conditions for different VCSEL-to-VCSEL injection locking configurations with respect to free-running case. We experimentally evaluated the capability of adjusting the frequency chirp, reducing its value and inverting the sign. The control over the frequency chirp is obtained changing the wavelength detuning and power injection ratio between the VCSEL master and the VCSEL slave. Advantages of the chirp inversion are demonstrated for 10 Gb/s error-free propagation at 1.55-μm over 40-km standard single mode fiber without any dispersion compensation.
© 2009 OSA
Nowadays, for optical communications vertical cavity surface emitting lasers (VCSELs) appear very interesting for their intrinsic characteristics, such as low power consumption, low cost, single mode emission and high efficiency . On the other hand, injection locking of semiconductor lasers is a well-known technique to improve laser performance for both digital and analog applications [2,3]. In particular, by employing VCSEL as slave source it has been demonstrated the capability of this technique to enhance laser resonance frequency [4–6], increasing direct modulation speed, reducing distortion, thus improving propagation performances.
In case of metropolitan applications, where the fiber chromatic dispersion is not optically compensated, the maximum reach is limited by the frequency chirp induced in the emitted laser source by the direct modulation. Negligible chirp is desirable, above all for 1.55-μm transmission. Frequency chirp reduction using optical injection locking has been shown both theoretically and experimentally for VCSEL sources . Moreover, it would be extremely useful to tune the frequency chirp both in magnitude and in sign in order to optimize optical propagation with respect to the uncompensated transmission link. Adjustable chirp has been studied in [7,8] in case of 1.55-μm VCSEL slave and distributed-feedback (DFB) source master, where low power penalties have been achieved after 100-km propagation over standard single mode fiber (SSMF) at 10 Gb/s.
For the first time, in this paper we experimentally demonstrate an efficient injection locking mechanism to adjust the frequency chirp value of the locked 10-Gb/s directly modulated VCSEL slave, by using also a single-mode VCSEL laser as master. Different operating conditions have been investigated and show the capability to reduce the frequency chirp value and also to invert its sign with respect to the free-running condition. This inversion is not achieved by the inversion of the bit pattern as described in . The performed 1.55-μm VCSEL-to-VCSEL injection-locking heavily improves propagation performance in uncompensated SSMF links. In particular, 10-Gb/s error-free propagation over uncompensated 40-km SSMF with negligible power penalty has been achieved and demonstrated.
In Session 2 we report the characterization of the VCSEL device we used in our experimentation, while in Session 3 the experimental setup employed to the chirp and propagation performances measurement is described. In Session 4 are shown the chirp measurements results in under-threshold and above-threshold operating conditions, showing the capability of the VCSEL-to-VCSEL injection locking to invert the sign of the chirp and reduce its value. Finally, in Session 5 very good improvement in 10-Gb/s transmission performance over 40-km SSMF link are demonstrated through eye diagrams and BER measurements.
2. 1.55-µm single-mode VCSEL characterization
The basic structure of the 1.55-µm single-mode VCSELs suitable for high speed applications employed in our experimentation is basically the same as described in [8,9] with optimized heat management in the cladding layers and improved bottom-mirror reflectivity.
The VCSEL master chip is mounted into a transmitter optical sub-assembly (TOSA) module with an LC-receptacle, including an optical isolator, a monitoring diode and a 50 Ω flex circuit connection, while the VCSEL slave chip is a pigtailed version. The optical single-mode spectrum shows a side mode suppression ratio beyond 40 dB over the relevant current and temperature range. The bandwidth exceeds 7.5 GHz at room temperature and it is limited by internal parasitics of the inner layers of the VCSEL and by thermal effects . However, this bandwidth enables data transmission at 10 Gb/s.
Linewidth measurement of 1.55 μm VCSEL device under test has been carried out in continuous-wave (CW) operation at different temperatures using the delayed self-heterodyne method with a decorrelation length of 70 m, which guarantees the operation in incoherent regime [11,12]. Figure 1 shows the linewidth vs. the reciprocal output power 1/P measured for 20°C, 40°C and 70°C. Both in linear regime and at higher power levels, the obtained linewidth is in agreement with the measured one in . For all temperature range taken into account the smallest linewidth over 1/P is achieved around output power of 0.5 mW and is about 7 MHz. The device shows very good linewidth characteristic, one of the best to our knowledge for long wavelength VCSEL.
3. Experimental setup
The schematic of the experimental setup for chirp measurements and for evaluating the propagation performance is shown in Fig. 2 . The master laser is a TOSA VCSEL with an integrated optical isolator to prevent any optical feedback from the slave VCSEL; the master emitted power is limited to 0 dBm owing to the coupling losses and to the integrated isolator. To evaluate the performance of the slave laser under different injection ratios an erbium-doped fiber amplifier (EDFA) and an optical attenuator after the master VCSEL are inserted. The EDFA is introduced to have the opportunity to change the injected power in order to find the best locking condition, but as shown below, the power useful for the locking is low, normally achieved by commercial VCSEL devices. The polarization controller is set to match exactly the polarization between the two lasers. The slave VCSEL is a pigtailed version with a current threshold of 2.3 mA and its maximum emitted power is about 2.2 dBm at a bias current of 17 mA. The injection-locked VCSEL is directly modulated by a PRBS 27-1 at 10.3 Gb/s. To investigate the injection-locking phenomenon, the slave wavelength is detuned with respect to the master one using a temperature controller. The injection power is changed through the EDFA and the attenuator and the amount of power injected into the slave VCSEL is measured at port 2 of the circulator.
As shown in Fig. 2(bottom), to perform chirp measures we employ a Mach-Zehnder interferometer (MZI) directly connected to the port 3 of the circulator. The delay τ between the two arms of the interferometer is calculated to obtain a Free Spectral Range (FSR) of 125 GHz, which allows to evaluate frequency deviation up to ± FSR/4. To achieve real time measurement, the two output ports of the MZI have the same length and are connected to two equal photoreceivers (20-GHz at 3-dB bandwidth). Each waveform is then acquired by a sampling oscilloscope and processed by a PC . The chirp of the injection locked slave is evaluated under different operating conditions, changing the wavelength detuning Δλ and the injection ratio ΔP. Then, to verify the injection locking effects over the modulated signal, we use the experimental setup shown in Fig. 2(top). The optical signal emitted by the injection locked VCSEL propagates over 40-km SSMF link after port 3 of the circulator. At the receiving end another EDFA is placed with an optical attenuator to perform BER measurement. The receiver is an avalanche photodiode (APD) with a sensitivity of −26 dBm at an extinction ratio of 10.2 dB. The spectra of the two lasers are monitored by a 50/50 coupler after port 3 of the circulator.
4. Frequency chirp measurements
To perform real-time chirp measurement we use the frequency discriminator method , using a Mach-Zehnder interferometer with a FSR = 125 GHz, that is a suitable value for high bit rate operation. The adiabatic chirp refers to a shift between the emission frequency of the two possible levels of the signal corresponding to the two logical bits, level ’0’ and level ‘1’. The transient chirp appears at the rising and falling edges of the signal as spikes. For this case, a “positive chirp” is observed where there is positive frequency change on the rising edge and negative frequency change on the falling edge of the data pattern. Positive chirp speeds up pulse broadening in case of transmission over SSMF with positive dispersion at 1.55 μm, increasing power penalty. We experimented two particular operating conditions, where the free-running VCSEL slave operates in under-threshold condition and in above-threshold condition.
4.1 Under-threshold operating condition
In Fig. 3 frequency chirp (blue line) and intensity (red line) measurements relative to a specific intensity pattern are shown for a free-running VCSEL directly modulated at 10.3Gb/s (Fig. 3(a)) and for injection-locked VCSEL in different conditions of wavelength detuning Δλ and injection ratio ΔP (Fig. 3(b), (c), (d)).
The device is biased at 6.6 mA and the amplitude modulation is 13 mA. In case of free-running operation (a) the chirp is positive, the adiabatic chirp value is about 5 GHz, while there is a strong transient component. The peak-to-peak chirp is about 21.3 GHz. In these conditions the chirp is too high and limits the propagation performance of the laser. As shown in (b), (c) and (d), thanks to injection locking the frequency chirp value is controlled by changing the power injection ratio and the wavelength detuning between master and slave. In all three different experimented configurations a good chirp reduction is achieved compared to the free-running condition. It is also visible the difference of the chirp sign. In Fig. 3(b) the injection ratio is 4 dB and Δλ = 0.27nm. These conditions lead to a peak-to-peak chirp value of 9 GHz, which remains positive. By decreasing the Δλ up to 0.24 nm, the peak-to-peak chirp value remains almost the same (8.6 GHz), but there is an inversion of the chirp sign (Fig. 3(c)). This inversion is clearly visible observing the adiabatic chirp: the frequency of the ‘0’ level is now higher than the frequency of ‘1’ level. Then, keeping the same Δλ, but decreasing the injection ratio up to 2 dB it is possible to achieve a condition where the adiabatic chirp is null (Fig. 3(d)), while the peak-to-peak value is 7.46 GHz. It is interesting to notice that the extinction ratio (ER) is improved when the chirp is inverted: with the positive chirp we obtain ER = 1.8 dB, while in the inverted condition we obtain ER = 3.18 dB. By increasing the injection ratio, the ER decreases, so it is fundamental to find a tradeoff between the chirp and ER of the signal. The performed chirp inversion is achieved with low injection ratio, demonstrating the possibility to employ two VCSELs with standard output power and no EDFA is necessary for a in-field application.
4.2 Above-threshold operating condition
In Fig. 4 frequency chirp (blue line) and intensity (red line) measurements relative to a specific intensity pattern are shown for a free-running VCSEL directly modulated at 10.3Gb/s (Fig. 4(a)) and for injection-locked VCSEL in case of Δλ = 0.22 nm and ΔP = 3.8dB (Fig. 4(b)). The device is biased at 8.5 mA and the amplitude modulation is 11.2 mA, hence a low value of free-running ER (about 3 dB) is obtained. It is clearly visible in Fig. 4(a) that the chirp of the free-running slave has the same shape with respect to the analogous condition under threshold (Fig. 3(a)), while it is reduced in magnitude. In fact, the peak-to-peak chirp is about 15 GHz, while the adiabatic chirp remains the same about 5 GHz. The reduction of the peak-to-peak chirp is due to the reduction of the transient component, that is expected with a low ER. However, also in this condition the chirp is positive and too strong to permit a 1.55-μm propagation over few km of SSMF.
In the injection locked condition the chirp reduction is clearly visible: the adiabatic chirp is around negligible and the peak-to-peak chirp value is 5.97 GHz. The transient component seems to be inverted respect to the free-running condition, i.e. the chirp is negative. This condition is achieved with very low ER (about 1.3 dB), because of the low initial ER (3 dB) and the high injection ratio (> 4 dB). Low injection ratios lead to lower chirp reduction and no inversion can be reached. We notice that this locking configuration is more unstable with respect to the under threshold locking condition.
We check also the behavior of other VCSELs with the same features and we achieved the same operation results, confirming that the performed chirp inversion is not due to the particular employed device.
5. Propagation performance at 1.55 μm over 40-km SSMF
Our experimentation shows that the optimal injection-locking condition is achieved when the detuning between the master and the slave wavelengths is + 0.24 nm with the master VCSEL on the red side (longer wavelength). This condition is reached with a master injected power about 2.1 dBm (circulator loss included), measured at port 2 of the circulator, and a free-running emitted power by the slave VCSEL of −2.2 dBm. The injection ratio is about 4 dB, so the optimal operation point can be reached also without the EDFA using a more powerful master. The ER of the modulated signal at port 3 of the circulator is about 3.18 dB. In order to demonstrate the benefit of the experimented inversion of the frequency chirp, the performance of the injection locked VCSEL are measured after 40 km propagation of SSMF link (chromatic dispersion at 1.55 μm D = + 16 ps/nmkm). In Fig. 5 the eye diagrams of the modulated signal are shown before and after propagation. The free-running back-to-back eye diagram (Fig. 5(a)) is clearly open, even if we work with a low extinction ratio with respect to its optimal operation conditions. After 40-km SSMF propagation the eye is totally distorted owing to the high accumulated dispersion (640ps/nm) (Fig. 5(b)). In the injection locking regime, the back-to-back eye (Fig. 5(c)) is open even if is visible a noise contribution probably due to the EDFA located after the master laser. After 40-km propagation (Fig. 5(d)) the eye is still open. Notice that the detected eye diagrams appear in this case with different rising/falling edge slopes with respect to the free-running case thanks to the frequency chirp inversion.
BER measurement as a function of received power is shown in Fig. 5(e). In back-to-back configuration at BER = 10−6 a power penalty of 0.5 dB is visible between the measures with and without injection locking. This starting penalty is probably due to the presence of ASE noise after master laser and before the receiver. After 40-km SSMF propagation is noticed a 0.5-dB improvement respect to back-to-back condition (at 10−6 BER) that confirms the chirp inversion. Error-free transmission is achieved after 40-km propagation without any dispersion compensation.
For the first time we demonstrated frequency chirp adjustment by means of VCSEL to VCSEL injection locking at 1.55 μm in case of 10-Gb/s modulation bit-rate, by changing the injection ratio and the wavelength detuning between the master and the slave. The experimentally achieved chirp inversion permits to overcome the limitations in the propagation performances due to the large spectrum induced by the direct modulation. Error free propagation over 40-km uncompensated SSMF at 1.55 μm is obtained. The experimented solution employing VCSEL sources not only as slave but also as master appears an interesting and low-cost approach to achieve very good propagation performance without employing DFB lasers in different applications, such as metropolitan links. Further investigation on frequency chirp behaviour in VCSEL-to-VCSEL injection locking, also from a theoretical point of view, is carrying out.
References and links
1. A. Gatto, A. Boletti, P. Boffi, C. Neumeyr, M. Ortsiefer, E. Rönneberg, and M. Martinelli, “1.3 μm VCSEL Transmission Performance up to 12.5 Gb/s for Metro Access Networks,” IEEE Photon. Technol. Lett. 21(12), 778–780 ( 2009). [CrossRef]
2. J. M. Liu, H. F. Chen, X. J. Meng, and T. B. Simpson, “Modulation bandwidth, noise, and stability of a semiconductor laser subject to strong injection locking,” IEEE Photon. Technol. Lett. 9(10), 1325–1327 ( 1997). [CrossRef]
3. V. Annovazzi-Lodi, A. Scire, M. Sorel, and S. Donati, “Dynamic behavior and locking of a semiconductor laser subjected to external injection,” IEEE J. Quantum Electron. 34(12), 2350–2357 ( 1998). [CrossRef]
4. L. Chrostowski, C.-H. Chang, and C. J. Chang-Hasnain, “Enhancement of Dynamic range in 1.55μm VCSELs using Injection Locking,” IEEE Photon. Technol. Lett. 15(4), 498–500 ( 2003). [CrossRef]
5. L. Chrostowski, B. Faraji, W. Hofmann, M.-C. Amann, S. Wieczorek, and W. W. Chow,, “40 GHz Bandwidth and 64 GHz Resonance Frequency in Injection-Locked 1.55μm VCSELs,” IEEE J. Quantum Electron. 13(5), 1200–1208 ( 2007). [CrossRef]
6. C.-H. Chang, L. Chrostowski, and C. J. Chang-Hasnain, “Injection locking of VCSELs,” IEEE J. Quantum Electron. 9(5), 1386–1393 ( 2003). [CrossRef]
7. B. Zhang, X. Zhao, L. Christen, D. Parekh, W. Hofmann, and C. Ming, Wu, M. C. Amann, C.J. Chang-Hasnain, and A. E. Willner, “Adjustable Chirp Injection-Locked 1.55μm VCSELs for Enhanced Chromatic Dispersion Compensation at 10 Gb/s,” in Proceedings of IEEE Conference on Optical Fiber Communication (San Diego Convention Center, 2008).
8. D. Parekh, and X. Bo Zhang, Zhao, Y. Yue, W. Hofmann, M.C. Amann, A.E. Willner and C.J. Chang-Hasnain, “90-km single-mode fiber transmission of 10-Gb/s multimode VCSELs under optical injection locking,” in Proceedings of IEEE Conference on Optical Fiber Communication (San Diego Convention Center, 2009).
9. M. Ortsiefer, W. Hofmann, E. Ronneberg, A. Boletti, A. Gatto, P. Boffi, J. Rosskopf, R. Shau, C. Neumeyr, G. Bohm, M. Martinelli, and M. C. Amann, “High speed 1.3 μm VCSELs for 12.5 Gbit/s optical interconnects,” Electron. Lett. 44(16), 974–975 ( 2008). [CrossRef]
10. M. Ortsiefer, M. Grau, J. Rosskopf, R. Shau, K. Windhorn, E. Rönneberg, G. Böhm, W. Hofmann, O. Dier, and M.-C. Amann, “InP-based VCSELs with Buried Tunnel Junction for Optical Communication and Sensing in the 1.3-2.3 μm Wavelength Range,” in Proceedings of IEEE Semiconductor Laser Conference, (Waikoloa, HI, USA, 2006), pp. 113–114.
11. W. Schmid, C. Jung, B. Weigi, G. Reiner, R. Michalzik, and K. J. Ebeling, “Delayed self-heterodyne linewidth measurement of VCSELs,” IEEE Photon. Technol. Lett. 8(10), 1288–1290 ( 1996). [CrossRef]
12. Dennis Derickson, Fiber Optic Test and Measurement (Prentice Hall,).
13. R. Shau, H. Halbritter, F. Riemenschneider, M. Ortsiefer, J. Rosskopf, G. Bohm, M. Maute, P. Meissner, and M.-C. Amann, “Linewidth of InP-based 1.55 lm VCSELs with buried tunnel junction,” Electron. Lett. 39(24), 1728 ( 2003). [CrossRef]
14. C. Laverdière, A. Fekecs, and M. Tetu, “A New Method for Measuring Time-Resolved Frequency Chirp of High Bit Rate Sources,” IEEE Photon. Technol. Lett. 15(3), 446–448 ( 2003). [CrossRef]