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All-optical radio frequency conversion is proposed by directly modulated optically injection-locked vertical-cavity surface-emitting lasers. The enhancement effect of second order products of RF signals by OIL technique is analyzed based on reflection-mode OIL model. Simulation results show that high injection ratio and large wavelength detuning of OIL condition lead to a high RF conversion gain. Compared with free running condition, more than 20 dB RF conversion gain enhancement is achieved in the simulation. The experimental results of the RF conversion gain improvement ( + 18 dB) by OIL show excellent agreement with our simulation results. The spurious free dynamic range improvement ( + 15 dB) of conversion signals by OIL is also experimentally demonstrated.
©2013 Optical Society of America
1. Introduction
Optical injection locking (OIL) of semiconductor lasers has been demonstrated as an efficient and robust technique to improve the modulation performance of a directly modulated diode laser [1–3]. Recently, a reflection-mode OIL model is established to explain data pattern inversion of directly modulated (DM) OIL vertical-cavity surface-emitting lasers (VCSELs) with on-off keying (OOK) modulation [4]. As the wavelength detuning (Δλ = λMaster-λSlave) increases, the output data pattern changes from normal, to transitional, and finally an inverted state. The inverted data pattern results in an inverted chirp, which provides significant advantages for DM-VCSELs with drastically increased fiber transmission distance [5, 6]. These three output data patterns (normal, transitional, and inverted) indicate that the nonlinear characteristic of DM-VCSEL’s transfer function curve is enhanced by OIL technique. For this reason, DM-OIL-VCSEL can work as an optical frequency up/down-conversion mixer under some specific locking conditions. All-optical radio frequency (RF) conversion has been demonstrated by several methods with optical interferometer and photodiode [7] or directly modulated VCSEL [8]. If we compare these two methods, the second one costs less and has a simpler structure. However, we recently demonstrated RF down-conversion based on DM-OIL-VCSEL [9], which brings more conversion gain compared with the second method [8]. Also, the OIL-VCSEL has a high frequency response (>100 GHz) property [2, 10]. This property could potentially broaden frequency conversion range. In this paper, the work of RF conversion is significantly expanded compared with our previous work [9]: Firstly, the RF up-conversion is analyzed. Besides, the OIL parameters are optimized theoretically and experimentally. Moreover, RF up/down-conversion gain improvement ( + 18 dB) by OIL is experimentally demonstrated. Last but not the least, the SFDR improvement ( + 15 dB) of conversion signals by OIL is experimentally demonstrated. This paper proves that high frequency conversion gain, large SFDR of conversion signals and broad frequency conversion range of this novel method will make it an excellent candidate for frequency conversion in fiber-optic systems.
2. Theoretical modeling and simulation
The simulation of RF conversion by DM-OIL-VCSEL is based on a model including the interference effect of master laser reflection [4]. The light of master laser is divided into two parts. One part transmits into the cavity of the slave laser, and the other part is reflected by the front facet of the slave laser. The flow diagram of the simulation is shown in Fig. 1 . The field magnitude Ar and phase shift φr of the light reflected by the front facet of the slave laser can be solved based on DBR structure calculation, which is approximately Ainj and π. The output As from cavity is calculated with standard OIL rate equations [3]. The total output field At is the vector sum of As and the reflection of the master laser Ar.
Fig. 1 The flow diagram of the simulation: (a) Setting of the input modulated current J(t) (in electrons/sec.). (b) Calculation of the slave laser’s output field As from cavity based on standard OIL rate equations. (c) Calculation of the total output optical field At = As + Ar based on refection-mode OIL model. (d) Calculation of the output spectrum by Fast Fourier Transformation (FFT).
In the simulation, two-tone electrical signals at different carrier frequencies 3 GHz and 3.5 GHz (0 dBm for each tone) are combined together to directly modulate VCSEL under free running (FR) and OIL conditions. VCSEL is biased at 5.5 mA with 0.8 mW output power. To achieve high RF conversion gain, OIL parameters are optimized at 0.84 nm wavelength detuning and 17.7 dB injection ratio (Rinj = PMaster/PSlave). Figure 2 shows the output electrical spectra and waveforms under these two conditions. Based on Fig. 2(a), RF conversion gains are calculated as is shown in Table 1 . It seems that OIL technique can contribute more benefits to RF conversion gain ( + 18~20 dB) than RF gain ( + 10 dB). Figure 2(b) shows the output waveforms in time domain. Compared with the FR condition, output waveform is inverted under OIL condition because the large red detuning is assumed in our simulation [4].
Fig. 2 Simulation results of VCSEL’s output electrical frequency spectra and waveforms under two frequency tones (3.0 GHz and 3.5 GHz) modulation. (a) Frequency domain electrical spectra. (b) Time domain waveforms.
Table 1. Simulation Results of Conversion Gain Comparison between FR and OIL Conditions
2.1 Wavelength detuning (Δλ) optimization
In this simulation, injection ratio is fixed at 17.7 dB, and wavelength detuning is increased from −0.058 nm to 0.85 nm which is corresponding to the locking range. The RF up/down-conversion gains are calculated which are shown in Fig. 3 . For the blue-dashed curve of RF gain under OIL condition, there is an obvious dip around 0.25 nm wavelength detuning. This is because the transitional state is reached under OIL, which is explained by our reflection-mode OIL model [4]. The RF conversion gain is also very low under this specific condition. On the one hand, the locking condition is chosen on the red edge of the locking range to maximize the conversion gain. On the other hand, locking condition is not very stable on the red edge of locking range. Wavelength detuning is optimized due to the trade-off between these two at 0.84 nm, which represents the same condition of the simulation in Fig. 2. Up and down conversion gains based on OIL tend to be the same on the red edge of the locking range.
Fig. 3 Simulation results of DM-OIL-VCSEL’s RF conversion gains versus wavelength detuning when injection ratio is fixed at 17.7 dB. The gains of free running condition are also plotted in the black lines for comparison.
2.2 Injection ratio (Rinj) optimization
Figure 4 shows the RF up/down-conversion gain curves under different injection ratios (8.7 dB, 11.7 dB, 14.7 dB and 17.7 dB). The locking ranges of these four conditions are also labeled in Fig. 4. When wavelength detuning is fixed, conversion gain is not very sensitive to the injection ratio. Simulation results show that only less than 5 dB conversion gain improvement is obtained when injection increases from 8.7 dB to 17.7 dB. However, a high injection ratio leads to a large locking range, and large red detuning is critical for achieving a high up/down-conversion gain. Also, a higher injection ratio makes locking condition more stable when detuning is fixed. In brief, to obtain a maximum RF conversion gain, the higher injection ratio, the better.
Fig. 4 Simulation results of DM-OIL-VCSEL’s RF conversion gains versus wavelength detuning when injection ratios are 8.7 dB, 11.7 dB, 14.7 dB, and 17.7 dB respectively. The gains of free running condition are also plotted in the black lines for comparison.
2.3 Gain improvement by OIL
Figure 5 shows the gain comparison between FR and OIL condition after optimization of wavelength detuning. Compared with RF gain improvement, OIL technique brings more RF conversion gain. With each increased 10 dB of injection ratio, RF up/down-conversion gain increases ~16 dB and RF gain only increases ~7 dB. Gain improvement is only limited by injection ratio based on our simulation results.
Fig. 5 Simulation results of DM-OIL-VCSEL’s RF conversion gains versus injection ratio when wavelength detuning is optimized. The gains of free running condition are also plotted in the black lines for comparison.
3. Experimental results
Figure 6 shows the experimental setup. As a master laser, a high power tunable laser injection-locks the VCSEL through an optical circulator. Two sinusoidal signal generators are combined together by electrical combiner to directly modulate OIL-VCSEL. One signal generator works as a local oscillator (LO) and the other one generates RF signal.
Fig. 6 Experimental setup for optical frequency up/down-conversion by OIL-VCSEL. (VCSEL: vertical cavity surface emitting laser, OC: optical circulator, PC: polarization controller, EC: electrical combiner, PD: photodiode, OSA: optical spectrum analyzer).
3.1 Gain improvement by OIL
In the experiment, the carrier frequencies of the LO signal and RF signal are 3.0 GHz and 3.5 GHz respectively, and the powers are both 0.0 dBm at output port of electrical combiner (EC). The parameter setting here is consistent with our simulation condition before. Figure 7(a) shows the electrical spectrum of output signal under FR condition. VCSEL is biased at 6.5 mA with 0.5 mW output optical power. Under this condition, the optical spectra of VCSEL with and without signal modulation are plotted with black solid and dotted curves in Fig. 7(c). With modulation, the spectrum is broadened to ~0.75 nm. Under OIL condition, the OIL parameters are optimized at 0.57 nm wavelength detuning (λmaster = 1532.09 nm, λslave = 1531.52 nm) and 18.0 dB injection ratio (PMaster = 15.0 dBm, PSlave = −3.0 dBm) to achieve high RF conversion gain. Figure 7(b) shows the electrical spectrum of output signal. Under this condition, the optical spectra of OIL-VCSEL with and without signal modulation are plotted with blue solid and dotted curves in Fig. 7(c). With OIL, the spectrum is narrowed to ~0.25 nm, which effectively conquers chromatic dispersion in fiber transmission. Based on Fig. 7(a) and 7(b), RF conversion gains are calculated as is shown in Table 2 . Comparison between Table 1 and Table 2 shows that the simulation results agree with our experimental results.
Fig. 7 Experimental results of VCSEL’s output electrical frequency spectra and optical spectra under two frequency tones (3.0 GHz and 3.5 GHz) modulation. (a) Electrical spectrum under free running condition. (b) Electrical spectrum under optimized optical injection locking condition. (c) Optical spectra under different conditions.
Table 2. Experimental Results of Conversion Gain Comparison between FR and OIL Conditions
Figure 8 shows the gain comparison between FR and OIL condition after optimization of wavelength detuning. Gain improvement is only limited by the maximum output power of the master laser (Santec TSL210) in our experiment.
Fig. 8 Experimental results of DM-OIL-VCSEL’s RF conversion gains versus injection ratio when wavelength detuning is optimized. The gains of free running condition are also plotted in the black lines for comparison.
3.2 Spurious free dynamic range (SFDR) of frequency conversion signals
In this part, another RF signal generator and electrical combiner are added into the original experimental setup to test spur-free dynamic range (SFDR) of the output up/down-conversion signals. The updated experimental setup is shown in Fig. 9 .
Fig. 9 Experimental setup for SFDR testing. (VCSEL: vertical cavity surface emitting laser, OC: optical circulator, PC: polarization controller, EC: electrical combiner, PD: photodiode, OSA: optical spectrum analyzer, ESA: electrical spectrum analyzer).
In the experiment, the carrier frequencies of the two RF signals are 4.99 GHz and 5.01 GHz respectively. LO signal is at 4.9 GHz with 4.7 dBm output power. VCSEL is biased at 6.0 mA with 0.4 mW output optical power. Under OIL condition, the OIL parameters are optimized at 0.73 nm wavelength detuning (λmaster = 1532.60 nm, λslave = 1531.87 nm) and 16.7 dB injection ratio (Pmaster = 12.9 dBm, Pslave = −3.8 dBm). Figure 10 shows a third-order intermodulation distortion (IMD3) limited SFDR comparison between the FR and OIL condition for both RF up/down-conversion. Compared with the FR condition, OIL deteriorates the IMD3 of up/down-conversion signals, but improves conversion gain, which is the main influential factor in SFDR of up/down-conversion signals. For RF down-conversion, 15 dB improvement (SFDRDown-FR = 66 dB/Hz2/3, SFDRDown-OIL = 81 dB/Hz2/3) in SFDR is attained by OIL technique, as is shown in Fig. 10(a). For SFDR of up-conversion signals testing, similar results are obtained shown in Fig. 10(b).
Fig. 10 Experimental results of SFDR testing (RF signals: 4.99 GHz and 5.01 GHz, local oscillator: 4.9 GHz): (a) 90 MHz and 110 MHz down-conversion signals under FR condition (gray dotted lines) and OIL condition (blue solid lines). Insets: Electrical spectra for SFDR testing. (b) 9.89 GHz and 9.91 GHz up-conversion signals under FR condition (gray dotted lines) and OIL condition (blue solid lines).
Figure 11 shows the comparison between the down-conversion signals (blue solid lines) under OIL condition and RF signals (gray dotted lines) under FR condition. There is no SFDR deterioration of down-conversion signals by OIL technique compared with RF signals under FR condition.
Fig. 11 Experimental results of SFDR testing (RF signals: 4.99 GHz and 5.01 GHz, local oscillator: 4.9 GHz): 90 MHz and 110 MHz down-conversion signals under OIL condition (blue solid lines), 4.99 GHz and 5.01 GHz RF signals under FR condition (gray dotted lines). Insets: Electrical spectra for SFDR testing.
4. Discussion
OIL has been demonstrated as a very effective technique to enhance small-signal response [1, 2]. With strong OIL, resonance frequency > 100 GHz and 3-dB bandwidth up to 80 GHz have been demonstrated [2]. Many high frequency response applications have been demonstrated by OIL technique [10, 11]. However, all of these high frequency applications are based on narrow band modulation. There is no high speed base band application of OIL technique to the best of our knowledge. One possible reason is that second order signal enhancement by OIL is much greater than RF signal enhancement based on our analysis in this paper. The results contain guiding significance on OIL technique applications.
5. Conclusion
All-optical RF conversion of microwave signals is proposed by DM-OIL-VCSEL. Simulation results based on reflection OIL model reveal second-order product enhancement by DM-OIL-VCSEL. Conversion gain improvement ( + 18 dB) and SFDR improvement ( + 15 dB) of conversion signals by OIL technique are experimentally demonstrated. This novel method contains numerous remarkable characteristics: large SFDR of conversion signals, high frequency conversion gain and broad frequency conversion range.
Acknowledgments
This work was supported by the National Basic Research Program of China (973 Program 2012CB315606 and 2010CB328201). The authors thank Prof. Connie J. Chang-Hasnain and PhD candidate Weijian Yang (University of California at Berkeley) for stimulating discussions. The authors thank Ms. Rongrong Gu for correcting the English manuscript.
References and links
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