This paper presents a new technique to generate microwave signal using an electro-absorption modulator (EAM) integrated with a distributed feedback (DFB) laser subject to optical injection. Experiments show that the frequency of the generated microwave can be tuned by changing the wavelength of the external laser or adjusting the bias voltage of the EAM. The frequency response of the EAM is studied and found to be unsmooth due to packaging parasitic effects and four-wave mixing effect occurring in the active layer of the DFB laser. It is also demonstrated that an EA modulator integrated in between two DFB lasers can be used instead of the EML under optical injection. This integrated chip can be used to realize a monolithically integrated tunable microwave source.
©2009 Optical Society of America
Optical generation of microwave signal becomes more and more attractive due to its many advantages, such as low power consumption, low cost, high reliability , and without frequency bandwidth limitation of electronic components compared with electric solutions. This technique has been extensively exploited in broad-band wireless access, sensor networks, radar, satellite communication systems and some other commercial applications areas [2–6].
As the development of the photonics technologies, several techniques for generating microwave signal have been proposed [7, 8]. The microwave can be generated based on the modulation technique . After intensity modulation, a series of sideband modes appear and the frequency spacing between the adjacent modes is exactly equal to the modulation frequency. The beat signal at a frequency single or multiple to the modulation frequency can be obtained from the output of the photodetector. In this method, a broadband modulation lightwave source is required, and the frequency bandwidth of the generated microwave signal depends on the frequency responses of both lightwave source and photodetector. Two light beams from two laser sources can be used to generate microwave signal based on optical heterodyne. However, the light beams from different sources are frequency incoherent. Thus, the generated microwave signal has larger phase noise, and its linewidth depends on the optical spectral linewidth of the two light beams. Optical phase-locked loop (OPLL) [10–13] or optical injection locking [14–17] has been used to reduce the phase noise and stabilize the microwave frequency. Another technique is using a laser with dual longitudinal modes, and the frequency of the generated microwave signal can be adjusted by wavelength tuning [18–23]. A tunable laser combined with delayed interferometer can also be utilized to generate frequency-tunable microwave signal. In the above mentioned methods, a photodetector is required, and external fiber links between the laser and photodetector will introduce additional linewidth expansion and frequency instability.
In this paper, a new method for generating microwave signal in an EAM is proposed. Section 2 presents the experiment investigation using an electro-absorption modulator integrated with a DFB laser (EML) subject to optical injection. The resonance which is due to the four-wave mixing effect occurring in the active layer of the DFB laser is investigated in Section 3. Section 4 gives a simple method to estimate the frequency response of the modulator from the measured frequency response as photodetector. Section 5 presents the dependence of the generated microwave signal on the bias voltage applied to the modulator. In Section 6, the microwave generation using an EA modulator integrated with two DFB lasers is demonstrated, and followed by a summary in Section 7.
2. Microwave generation based on optical injection
Figure 1 shows the experimental setup for microwave signal generation using an EML, which produces one light beam. Another light beam was from a narrow-linewidth tunable laser (Agilent 81600B), and injected into the EAM through an optical circulator. These two beams mixed in the modulator. It has been observed that a reversely biased EAM can be utilized as high-frequency photodetector [24–28]. Therefore, a microwave signal can be generated in the modulator. The frequency of the generated signal exactly depends on the wavelength difference, and the power can be expressed as .
In the experiments, the EAM was biased through a bias Tee and the generated microwave signal was measured by an electrical spectrum analyzer (Advantest R3182). The output from port 3 of the optical circulator consists of the lightwaves from the DFB laser and the tunable laser. The mixed lightwaves are split into two waves by an optical fiber coupler. One beam is launched into an optical spectrum analyzer (Advantest Q8384) for measuring the optical spectrum. Another is for electrical spectral measurements using another spectrum analyzer (R&S FSP) with a high-speed photodetector (DSC 10ER). In this way, the spectrum of the microwave signals generated in both the EAM and the high-speed photodetector can be measured simultaneously.
The EML used in the experiment was fabricated in our lab using a two-step lower-pressure metal–organic vapor phase epitaxial process. Figure 2(a) shows the cross-section structure diagram of the EML. Firstly, a pair of SiO2 masks for selective area growth (SAG) was patterned on the substrate. The multiple quantum well and separate confinement heterojunction layer were then grown in the first epitaxial growth. The MQW structure consists of ten compressively strained InGaAsP quantum wells and nine lattice-matched InGaAsP barriers. The separate confinement heterojunction layers are fabricated on both sides of the MQW-layer. The SAG process created a 50-nm bandgap difference between the modulator and the DFB laser. A uniform grating was formed only in the laser section. A thin P-InP cladding layer and an InGaAs cap layer were then grown in the second epitaxial growth step. This was followed by conventional ridge processing of the DFB laser and EAM sections. The DFB laser and EAM sections were electrically isolated by etching away the highly conductive InGaAs cap layers between them and by subsequent He+ implantation in the trench. Standard P- and N-contacts were finally fabricated on the top and bottom sides. The integrated device was packaged in butterfly housing without optical isolator.
In the measurement, the DFB laser was biased at 60 mA and its wavelength was 1541.625 nm. The wavelength of the tunable laser was tuned to be close to that of the DFB laser, so that the wavelength difference is within 30 GHz. The EAM was biased at −0.8 V. After the attenuation of the modulator the optical output power measured at the pig-tail fiber of the EML was 39 μW. The injection optical power measured at port 2 of the optical circulator was 1.3 mW.
When the wavelength of the narrow-linewidth tunable laser was tuned to be 1541.686 nm, the wavelength difference is about 7.5 GHz. Figure 2(b) shows the spectrum of the microwave signal generated in the EAM. Figure 2(c) shows the spectrum of the beat signal between the two lightwaves generated in the high-speed photodetector.
Figure 3 shows the spectra of the microwaves generated in the EAM and in the high-speed photodetector at different injection wavelengths with a step of 2.5 GHz. In the measurement, the wavelength was positively detuned. It is obvious that the microwave signals generated in the EAM and the photodetector have the same frequencies but different magnitudes, since the spectrum measurements are done at the same time. The traces of the peaks also indicate the frequency responses of the EAM and high-speed photodetector .
Figure 4 gives the measured spectra when the injected optical wavelength was negatively detuned. Comparing Figs. 3 and 4 one can see that almost the same microwave signals can be obtained when the injected optical wavelengths are positively or negatively detuned.
Figure 5 shows the magnitudes of the microwave signal peaks detected by the EAM with different injection optical powers when the modulator is biased at −0.8 V and the injected optical wavelength is positively detuned at 7.5 GHz. As expected, the microwave power is linearly proportional to the injected optical power .
3. Effects of four-wave mixing in DFB laser
Figure 6 shows the measured peak values of the microwave signal generated in the EAM when the optical power of the tunable laser was kept constant. In the experiment, the wavelength of the DFB laser was fixed and the tunable laser was tuned to make the wavelength-difference of those two lightwaves sweep in a frequency range of 30 GHz. The trace of the beat signal peaks (closed circles) indicates the frequency responses of the EAM when it was used as a photodetector . It can be seen that the frequency response is not smooth, especially at lower frequencies.
In order to check the source of the resonance, the DFB laser was turned off and replaced by another external tunable laser (Agilent 81949A). The results (opened circle) are also plotted in Fig. 6. Comparing it (opened circle) with the result mentioned above (closed circles), one can clearly notice the difference at low frequency. It is believed that the low frequency resonance is due to four-wave mixing effect occurring in the active layer of the DFB laser. When the injected wavelength λinj (λ1) is tuned close to the DFB lasing wavelength λDFB, the four-wave mixing effect is much stronger, which results in a frequency response drop at lower frequencies, and the DFB laser may be locked to the injected wavelength.
When the EAM is used as a modulator, its frequency response (solid line) measured with a standard experimental arrangement using microwave network analyzer (Agilent 8720D) is also given in Fig. 6 for comparison. In this case the microwave signal from the network analyzer is applied to the EAM. The lightwave from the DFB laser is modulated through the modulator and detected by the photodetector. It is obvious that the four-wave mixing effect disappears when the DFB laser is turned off or there is no optical injection. From the measured responses shown in Fig. 6 one can also see that there are drops at about 15 and 20 GHz, which are due to the packaging parasitics of the EML.
Figure 7 shows the measured optical spectra at different optical injection wavelengths. A series of peaks appear due to four-wave mixing effect, and the wavelength spacing of the adjacent peaks is just the wavelength difference between the injected lightwave and the intrinsic lasing lightwave of the DFB laser. The amplitudes of the peaks decrease rapidly as the increase of the wavelength difference. This implies that four-wave mixing effect becomes much weaker at higher frequencies, and supports the statement that the weak response at low frequencies is due to the four-wave mixing effect occurring in the active layer of the DFB laser.
4. Frequency response estimation of EAM
It has been demonstrated in Section 3 that when two light beams are injected into an EAM, a microwave signal is generated in the modulator. The frequency of the generated microwave signal depends on the wavelength difference of the two beams. In this case the modulator is used as a photodetector. When the modulator is treated as a modulator, its frequency response can be directly measured using microwave network analyzer. From the measured results shown in Fig. 6 one can clearly see that when the modulator is used as a photodetector (opened circle) or a modulator (solid line), its frequency responses are almost identical. Therefore, the frequency response of the modulator can be estimated using the scheme shown in Fig. 1.
The intensities of the microwave signals generated in the EAM and the high-speed photodetector as functions of the modulator bias voltage are plotted in Fig. 8 , where the parameter is the wavelength difference. It is obvious that the generated microwave signals are stronger when the modulator is reversely biased at around 0.6 V. The results shown in Fig. 8(a) and (b) denote the responses of the EAM when it functions as photodetector and modulator at different reversely biased voltages, respectively. The similar curve shape indicates the same conclusion that the intensity of the generated microwave signal is proportional to the modulation depth . Therefore, the modulator can also be used to adjust the intensity of the generated microwave signal by varying the bias voltage.
5. Frequency tuning by varying bias voltage
The wavelength of the DFB laser can be shifted by adjusting the bias voltage of the EAM due to adiabatic chirp. When the isolation resistance between the integrated DFB laser and EAM is not large enough, the laser threshold current will vary with the reverse bias voltage. This leads to the laser wavelength shift, and can be used to finely tune the frequency of the generated microwave signal. Figure 9 shows the measured spectra of the generated microwave signals when the modulator is reversely biased at different voltages. 20 GHz wavelength shift can be obtained by changing the bias voltage from −0.6 to −2.0 V.
The experiment results show that the trace of the peaks indicates the similar unsmooth frequency response. When the EAM is biased at −2.0 V, the DFB lasing wavelength is close to the injected optical wavelength. Four-wave mixing effect occurring in the active layer of the DFB laser leads to weaker response at lower frequencies. One can also see the drops at about 15 and 20 GHz, which are caused by the packaging parasitics of the EML.
6. Microwave generation in monolithic integrated chip
Figure 10 gives the experimental setup for microwave generation using an EAM integrated in between two DFB lasers. The structures of the modulator and lasers are similar to those of the devices shown in Fig. 2(a). In this scheme, the wavelengths of the DFB lasers are tuned by adjusting their bias currents. The light beams from both DFB lasers are injected into the EAM and mixed with each other to generate microwave signal. The electrical spectra recorded using the function “Max hold” of the electrical spectrum analyzer show the maximal values in the observation time. A lensed fiber is used to monitor the optical wavelength change.
Figure 11 shows the optical and electrical spectra. In Fig. 11(a), four-wave mixing effect can still be observed when the optical wavelength difference is over 30 GHz due to strong optical coupling between the two lasers. From Fig. 11(b) it can be seen that a sharp peak at the beat frequency has a 24-dB signal noise ratio. The frequency of the generated microwave signal can be tuned by changing the bias currents of the DFB lasers. The results show that an EAM integrated in between two DFB lasers can be used as a monolithic integrated microwave source. The modulator in this scheme has three functions:
- 1) Generate microwave signal.
- 2) Control the intensity of the generated microwave signal.
- 3) Tune the frequency of the generated microwave signal.
It has been shown that an EAM can be used as a photodetector. In this study, the microwave signal is generated in the modulator which is integrated with a single or two DFB lasers. In the first experiment, the modulator is integrated with a DFB laser. One light beam is from the DFB laser and the other light beam is injected from a narrow-linewidth tunable laser. It has been shown that the amplitude of the generated microwave signal is proportional to the injected optical power and the modulation depth, and the wavelength of the DFB laser can be finely tuned by adjusting the bias voltage of the EAM. Therefore, the modulator can be used to adjust the amplitude and frequency of the generated microwave signal.
In the second experiment, the EAM is integrated in between two DFB lasers which have close but different wavelengths. It is demonstrated that the generated microwave signal has a 24-dB signal noise ratio, and its frequency can be tuned by adjusting the bias currents of the DFB lasers. This integrated chip can be used as a monolithically integrated tunable microwave source.
This work is supported in part by the National Natural Science Foundation of China under Grants 60820106004, 60536010, 60777029, and 60606019, and in part by the National Basic Research and High Technology Development Programs of China under Grants 2006CB604902, 2006CB302806, 2006dfa11880, and 2009AA03Z409
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