IEEE TRANSACTIONS PHOTONICS TECHNOLOGY LETTERS, VOL. zyxwvutsrqp 3, NO. 11, NOVEMBER, zyxwvuts 1991 1021 THz Optical-Frequency Conversion of 1 Gb/s-Signals Using Highly Nondegenerate Four- W ave Mixing in an InGaAsP Semiconductor Laser S. Murata, A. Tomita, J. Shimizu, and A. Suzuki Abstract-THz-range optical-frequency conversion of 1 Gb/s-signals is demonstrated, for the first time, through the use of cavity-enhanced highly nondegenerate four-wave mixing (HNDFWM) in an InGaAsP semiconductor laser. This conver- sion is based on a subpicosecond ultrafast nonlinear gain pro- cess in the laser. The possibility of applying this phenomenon to an optical fiber dispersion compensator is also discussed. OUR-WAVE mixing (FWM) in semiconductor lasers is F attractive for future optical device applications, such as a direct optical-frequency convertor in frequency-division pho- tonic systems zyxwvutsrqpo [ 11, [2] and a frequency-dispersion compen- sator in dispersive optical fiber transmission systems. Fre- quency convertors based on nearly degenerate FWM in trav- eling-wave semiconductor laser amplifiers have previously been demonstrated for the successful conversion of 140 Mb/s DPSK optical signals [3]. Since the nearly degenerate FWM in the experiment was due to carrier-density modulation, the detuning between the pump frequency and the input-data -signal (probe) frequency was less than several GHz and frequency conversion of Gb/s optical signals was not possi- ble. The authors have recently observed, however, highly nondegenerate four-wave mixing (HNDFWM) with more than 1 THz detuning in a 1.5 pm multiple-quantum-well distributed-feedback (DFB) laser [4]. HNDFWM is univer- sally observed in 1.5 pm InGaAsP lasers [5] and is based on an ultrafast intraband optical nonlinear gain process whose response time is less than 1 ps. It seems promising for application in wide-range frequency conversion of Gb/s opti- cal signals and for optical-fiber frequency-dispersion com- pensation. A dispersion compensator reverses the modulation signal spectrum in the frequency domain (spectrum inver- sion) midway in a dispersive transmission line. This results in the recovery at the end of the line of a degraded signal waveform [6]. In this letter, we report the application of the HNDFWM Manuscript received June 17, 1991; revised August 29, 1991. The authors are zyxwvutsrqpo with Opto-electronics Research Laboratories, NEC Cor- poration, Kawasaki 216, Japan. IEEE Log Number 9103808. process to a 1.5 pm InGaAsP semiconductor laser in optical- frequency conversion experiments on 1 Gb/s intensity-mod- ulated signals in a 1 THz conversion range. The HNDFWM was generated through the use of an injection-locking tech- nique. The converted signals were enhanced by laser-cavity resonance [7]. This is the first reported device application of the ultrafast nonlinear gain process in semiconductor lasers. The experimental setup is shown in Fig. 1. The 1.5 pm InGaAsP Fabry-Perot laser that we used for FWM was a compressive strained multiple-quantum-well laser composed of ten In,~,,Ga,,,,As wells (3 nm thick). Cavity length was 350 pm and threshold current was 12 mA at 50°C. A 1.5 pm DFB laser was used as a master laser. The probe light source consisted of a 1.5 pm DFB laser and a LiNbO, optical intensity modulator. The FWM laser, which was biased above the threshold, was injection-lockedby the master laser and operated in a single longitudinal mode. The lasing mode acted as a pump light in the HNDFWM process. The master light and the intensity-modulated probe light were injected into the FWM laser from the same facet. The converted signal was output from the other facet and separated by two Fabry - Perot-type optical-frequency fibers. The modulation waveform was monitored by an avalanche photodiode. The output spectrum for the FWM laser and the relative intensi- ties of the pump, probe, and converted-signal lights were measured with an optical spectrum analyzer. The frequencies of the master and probe lights were controlled by changing their temperature and driving currents. The three lasers were temperature-stabilizedwithin 0.01"C. The FWM laser output-spectra are shown in Fig. zyx 2. The bias current was 133 mA, which corresponded to 11 dBm output power when the FWM laser was free-running at 50°C. Fig. 2(a) indicates the spectrum when the FWM laser was injection-locked. The pump frequency was adjusted to the FWM laser gain peak. Fabry-Perot cavity resonance- modes were observed to occur either side of the pump frequency. Pump output power was 12 dBm. When the probe frequency was tuned to one of the resonance modes, a converted signal appeared at the corresponding resonant mode on the other side, as shown in Fig. 2@). The probe and converted-signal outputs were enhanced by the cavity reso- 1041-1135/91$01.00 01991 IEEE