1. Introduction

Precision and broadband frequency swept laser sources with high spectral purity are crucial tools in broad range of applications including coherent optical spectrum analyzer [1], ground-to-satellite optical Doppler ranging [2], synthetic-aperture imaging laser radar [3] and phase coherent optical communication [4]. However, for tunable laser source the linewidth will be invariably sacrificed to obtain agile frequency tuning performance [3, 5], and usual direct laser frequency sweeping techniques show inherent non-linearity in the frequency modulation response versus frequency control factors such as injection current, temperature, or mechanical displacement of the cavity, especially at high speed and large sweep ranges, which hampers precision and predictable optical frequency tuning. The simultaneous achievement of narrow linewidth operation and fast precision tuning is a challenge to conventional laser source.

Numerous methods have been presented to meet the required demands of the above mentioned applications. The most popular frequency precision tuning technique is to establish a phase-locking link between a tunable single-mode laser and an individual comb line of optical frequency comb (OFC) [6, 7] or a frequency stabilized laser [8] with desired frequency/phase offset using a tunable optical phase-locked loop (OPLL), this technique allows both line narrowing and frequency noise reduction of the tunable laser [9]. Precision tuning to and from any frequency within a 40 GHz tuning range is achieved by phase-locking a distributed Bragg reflector laser to a frequency-stabilized laser [8], moreover in recent research, an agile continuous tuning of the frequency/phase offset from OFC has been realized with an assist from a transfer interferometer [6], however these schemes always suffer from fine alignment, relatively costly implementation and complicated controlling electronics, and the tuning speed is limited by the OPLL. An alternative method is to modulate an ultra-stable laser by an acousto-optic modulator (AOM) or an electro-optic modulator (EOM) generating a frequency shift modulation-sideband, and the optical frequency shift can be tuned by precisely adjusting the radio frequency (RF) of the device driver electronics [10], an ultrafast frequency sweep of 11 GHz within 300 ns is realized with the help of the dual-parallel Mach-Zehnder modulator (DPMZM) driven by the arbitrary waveform generator (AWG) and the wideband frequency multiplier [11]. However, further improvement of the sweep range is limited by the bandwidth of the RF driver electronics and modulator devices.

Recently, Schneider et al. proposed a scheme utilizing the modulation-sideband-injection locking method to generate widely tunable radio frequency (RF) signals over seven octaves from a low-frequency reference RF signal [12], which shows the potential capacity of this method to produce an wideband continuous frequency swept laser. In this paper, we extend this modulation-sideband injection locking method into precision broadband high-coherence laser continuous frequency sweep and experimentally demonstrate it.

The intensity EOM driven by a RF signal is used to produce high-order sidebands. Because the sidebands and the carrier are mutually coherent with frequency offset from each other by multiples of the RF signal, so if the RF driven signal to the EOM is shifted by$\Delta \omega$, the n^th sideband frequency will shift instantly by$n\Delta \omega$, consequently the frequency sweep rate and range are simultaneously multiplied. Since the generating procedure of the sidebands is strictly bound to the RF signal and independent from the cavity reconfiguration, narrow linewidth of the sidebands are reserved even at high frequency sweep rate. A slave diode laser is injecting-locked to these high order sidebands. The injection-locked diode laser acts both as a frequency filter and an amplifier of a specific modulation-sideband to realize the frequency sweep and enhance the sideband suppression ratio (SBSR) and output power. A synchronous compensation current to the slave diode laser is implemented to maintain stable optical injection locking [13]. In this way, the frequency of the slave laser can be tuned in an extremely controlled and predictable manner, exactly the same as the specific modulation-sideband, and the slave laser also maintains the master laser’s narrow linewidth and good frequency stability during the entire frequency swept process. Meanwhile, this scheme offers a simple and effective method to extend the frequency sweep range and tuning rate, overcoming the restriction on the RF driver electronics [11, 12, 14–16].

2. Experimental setup

The configuration of the experiment setup used in this frequency swept laser source is shown in Fig. 1, and the callouts visualize the tuning concept. The master laser is a planar external cavity low noise laser (RIO ORION^TM laser module) with a typical 3 dB-linewidth of less than 3 kHz and 10 mW output power. The slave laser is a single-mode DFB-type butterfly-packaged diode laser, and it has no internal isolator to acquire high injection ratio. The slave laser is driven by an ultra-low noise current source (ILX Lightwave, LDX-3620B). An intensity EOM (Pholine, MXPE-LN-10) with bandwidth up to 12 GHz is used to produce the high order modulation-sidebands. Moreover the EOM is biased at the null point to suppress the carrier to eliminate its impact on injection locking [14]. The sidebands are launched into the free running slave laser through a circulator, and the slave laser can be locked to any sideband with sufficient power by tuning into the corresponding lock area, which is often in the negative frequency detuning range from the sideband during the injection-locking process. The frequency swept principle is visually illustrated in Fig. 1(a) and 1(b), the frequency of the sidebands can be conveniently tuned by changing the frequency of the driven RF signal, and a frequency shift of the RF driven signal $\Delta \omega$will introduce a frequency shift of n$\Delta \omega$ to the n^th sideband, as depicted in Fig. 1(a). A current compensation to the slave diode laser is implemented to maintain stable optical injection locking to the n^th sideband. In this way, the frequency of the slave laser can be swept by n$\Delta \omega$ using the n^th sideband, as depicted in Fig. 1(b).

 

Fig. 1 Schematic diagram of linearized frequency swept laser source based on modulation-sideband injection-locking and its performance testing equipments. Callouts indicate the spectra at various points throughout the system. RIO: master laser; EOM: electro-optic modulator; VOA: variable optical attenuator; DFB LD: distributed feedback diode laser; RF: radio frequency; PS: power splitter; VCO: voltage controlled oscillator; LPF: loop filter; OSA: optical spectrum analyzer; SM fiber: single mode fiber; AOM: acousto-optic modulator; PD: photodetector; SA: spectrum analyzer; OSC: oscilloscope.

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For the demonstration experiments, the EOM is driven by a wideband voltage controlled oscillator (VCO, Hittite, HMC-C029) covering 5.0~10.0 GHz with 20 dBm output power and low single side band (SSB) phase noise, and the output RF signal can be tuned continuously by analog tune voltage. For the intensity EOM, the conversion efficiency of the respective high order sideband depends on the driving power and signal property. The VCO's output RF signal itself contains multiple harmonics which is helpful for the generation of high order sidebands. To yield an even broader comb of sidebands with greater power in high orders, an additional RF amplifier (Pholine, DR-AN-10-MO) is used, its gain and saturated output power are 30 dB and 23 dBm respectively and it worked at saturated region, so that high order sidebands can be enhanced. In order to overcome the nonlinear tuning performance of the VCO, a fractional-N frequency synthesizer (ANALOG DEVICES, ADF4159) is utilized to assistant VCO for generating direct linearized modulated or arbitrary value output RF signal with low phase noise. A RF power splitter (PS) picks off a fraction of the RF signal before the RF amplifier as the input of the synthesizer. The synthesizer compares it with a 100 MHz reference from a temperature compensated crystal oscillator (TCXO) by a digital phase frequency detector (PFD). The output of the PFD is feedback to the VCO tune voltage through a loop filter (LPF) which forms a phase lock loop (PLL). RF signal spanning from 5 GHz to 10 GHz with hertz frequency resolution and good tunability can be obtained with this PLL synthesizer. Meanwhile, by properly amplifying the output voltage from the LPF, this signal can be used to synchronously adjust the current of the slave laser, making sure the frequency of the slave laser sweeps in tight with the sideband to be locked. However, it is worth noting that both VCO and diode laser exhibit nonlinearity during the frequency sweep, so there is an inevitable frequency deviation between the targeted sideband and the slave laser, but the deviation is always within the locking ranges in our experiments.

3 Experimental results and discussions

The output optical spectra of the EOM is measured by an optical spectrum analyzer (OSA) with a resolution of 0.04 pm (APEX Technologies, AP-2041B), as shown in Figs. 2(a) and 2(b). The RF signal is set to 5.5 GHz and 8.5 GHz respectively by the frequency synthesizer, a DC stabilized power supply provides a bias voltage to suppress the carrier. Sidebands at offset up to 60 GHz can be obtained by our experiment apparatus and parameters. Note that the side modes beside the 1st order sideband stem from the master laser.

 

Fig. 2 Optical spectra of the EOM output measured by optical spectrum analyzer with the RF signal of (a) 5.5 GHz and (b) 8.5 GHz respectively.

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The slave diode laser is injected directly by the modulation-sidebands through the circulator. Coarse frequency match is accomplished by thermal tuning of the slave laser, and further fine match is achieved by adjusting the drive current. To prevent instability in the slave laser caused by injecting too much power especially in the case of high order locking, the injected power must be reduced by a variable optical attenuator (VOA), and the injected ratio between the injected sideband power at the desired order and the slave laser free-running output power should be adjusted for optimal locking. Optical spectra of the slave laser injection-locked at 1st, 3rd, 5th order sideband with the RF signal set at 7 GHz are shown in Figs. 3(a)–3(c), the corresponding SBSR are also indicated. Note that similar results could be acquired with different RF frequency values. The rejection ratio is measured to be −52.37, −58.89, −59.53 dB by the OSA respectively. The SBSR is mainly affected by the injection ratio and the slave laser’s drive current. The deterioration of the SBSR in high order is mainly due to the relatively high injected power of the 1st order sideband, and it can be improved by inserting a sharp transition bandpass filter to suppress the 1st order sideband power in the future work. The locking area of the slave laser is reduced as the weakening of the injected power, however even in the case of injected ratio below −50 dB, the locking area is still measured about 500 MHz, which is wide enough to cover the variation induced by the frequency tuning nonlinearity of both the diode laser and VCO when the compensation current is applied.

 

Fig. 3 Experimentally measured optical spectra and linewidth of the injection-locked slave laser at 1st, 3rd, 5th order sideband.

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The linewidth of the injection locked slave laser is measured based on the fiber delayed self-heterodyne interferometric technique [17, 18]. The measurement equipment is shown in the left dashed box in Fig. 1, the fiber length is 20 km, and a frequency shift of 200 MHz is generated by an AOM. The output was detected by an amplified PIN photodiode receiver module (PD1, Optilab, LR-12-A-M, bandwidth of 12 GHz) and then sent into a spectrum analyzer (SA, Agilent, E4405B, 9 kHz~13.2 GHz). The results compared with master laser are shown in Figs. 3(d) and 3(e). The beat spectra in Fig. 3 (d) are acquired over 3 MHz centered on 200 MHz, with a RBW of 100 Hz, the sidelobe peaks offset by 1 MHz from the central frequency corresponds to the bandwidth of the LPF, which arises from the PLL electronic signal coupling into the RF signal amplifier–modulator chain. Figure 3(e) is a zoomed-in trace over a 1 MHz span with a 30 Hz RBW, showing the 20 dB-linewidth of the beat signal is 50 kHz, which corresponds to a Lorentz 3 dB-linewidth of ~2.5 kHz. The phase noise of the injection locked slave laser introduced by the PLL is enhanced as the locked sideband order increases, as we know that the phase noise scales with multiplication of RF signal harmonics by orders as 20log₁₀n [12]. Meanwhile, the phase noise of the injection locked slave laser is also affected by the injection ratio, and high injection ratio brings out better phase noise suppression. However, the power distributions around the beat central peak almost remain unchanged.

By injection locked at high order sideband, the slave laser can be fast and broadband swept by the PLL frequency synthesizer agilely. As shown in Fig. 1, the frequency synthesizer will utilize a feedback controlling voltage to the VCO’s tuning port through the loop filter of the PLL to control the output RF frequency, on the other hand, the slave laser’s current drive circuit has an external modulation port allows converting the external input voltage into the drive current variation. So the feedback controlling voltage can be added to the external tuning port after the variable gain amplifier, and the amplification factor should be adjustedaccording to the different sideband to make sure the slave laser sweeping and locking to the target sideband. For a demonstration, the slave is locked at the 1st order and 5th order sideband respectively, while the frequency synthesizer is set to sweep from 5.5 GHz to 8.5 GHz with the step of 10 kHz. The frequency sweep of the slave laser can be derived from a fiber asymmetric Mach-Zehnder interferometer, as shown in the right dashed box in Fig. 1. The length of the fiber delay line denoted by ΔL₁ used here is~25 m. For the small steppingvalues, the laser frequency stepping can be seen as quasi-continuous laser frequency sweep, in that case, the output power variation with time of the interferometer exhibits consecutively sinusoidal curve which is denoted by y(t), and the instantaneous phase of the generated photocurrent denoted by Δφ₁ is proportional to the frequency chirp [19, 20]. The instantaneous equivalent phase can be acquired by the Hilbert transform H{y(t)} using the equation written by tan⁻¹[y(t)/ H{y(t)}], which shows as a sawtooth waveform. The absolute instantaneous phase variation Δφ₁ can be obtained by adding a correction phase difference of π at each turning point of the sawtooth waveform, and the swept frequency with respect to time can be determined by cΔφ₁/(2πnΔL₁), where c is the speed of light in a vacuum, and n is the refractive index of the fiber core. Therefore, the linearity and the range of the frequency sweep can be assessed using the Hilbert transformation. The instantaneous optical frequency as a function of time and the residual errors from a linear fit are shown in Fig. 4(a) and Fig. 5(a). The optical frequency of our laser is observed to remain linear sweep to within a standard deviation of 165 kHz throughout a 3 GHz chirp in 6 ms with the 1st order sideband injection-locking from Fig. 4(a) and a standard deviation of 240 kHz throughout a 15 GHz chirp in 6 ms with the 5th order sideband injection-locking from Fig. 5(a). The sweep range with the high-order sideband injection-locking is widely expanded compared with the 1st sideband injection-locking. The spikes on the residual errors are supposed to be introduced by the environment disturbance in the laboratory. The insets within Fig. 4(a) and Fig. 5 (a) are Fourier transforms of the photocurrent, show narrow peaks at the frequency of 61.9 Hz and 309.7 Hz, which indicates that the tuning rate in Fig. 5(a) are five times of that in Fig. 4(a), in perfect accordance with the setting value by frequency synthesizer [21]. Note that the width of the spectrum peak with tuning rate of 0.5 THz/s are broadened by low frequency noise, but still limited to sub-kilohertz.

 

Fig. 4 (a) Instantaneous optical frequency changes as a function of time and the residual errors from a linear fit with 1st order sideband injection-locking, the insets are enlarged Fourier transforms of the interferometer outputs; (b) linewidth comparisons of the laser with linearized swept-frequency and fixed-frequency operation.

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Fig. 5 (a) Instantaneous optical frequency changes as a function of time and the residual errors from a linear fit with 5th order sideband injection-locking, the insets are enlarged Fourier transforms of the interferometer outputs; (b) linewidth comparisons of the laser with linearized swept-frequency and fixed-frequency operation.

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The tuning rate and the coherence of the tuning laser can also be evaluated by the aforementioned linewidth measurement equipment. For the length of the delay fiber is ΔL₂ (which used here is 20 km), corresponding to a delay time of τ₂ = nΔ₂L/c (which is 0.1 ms here), and assume the laser tuning rate is v(t), then in the instance of the ideally linearized frequency sweep, the interference output is dominated by the average effect of the beat signal of two incoherent narrow linewidth lasers with frequency difference of v(t)τ₂ (which is 50MHz and 250MHz in here) during the whole frequency chirp. Moreover the linewidth of the electric spectrum can demonstrate the spectral purity and the tuning linearity of the chirped laser, and the tuning rate can be acquired by the central frequency deviation. The measurement results are shown in Fig. 4(b) and Fig. 5 (b), corresponding to the different tuning rate in Fig. 4(b) and Fig. 5 (b) respectively, and are compared with the linewidth of slave laser locked at 1st order and 5th order sideband with RF signal set at 7 GHz. The spectrum shapes of locked laser at the fixed and swept RF signal are generally the same, but the swept lasers have a frequency shift at the spectrum center (the frequency shift is measured to be 150.27 MHz and 48.6 MHz in Fig. 4(b) and Fig. 5 (b)), which is in perfect agreement with the previous analysis when the 200MHz frequency of the AOM is subtracted. The experimental results show the injection locked slave laser is tuned with high optical spectral purity, and meanwhile indicate that the frequency tuning rate maintains a fixed value and the frequency sweeps with high linearity. This present implementation is just an early stage demonstration and only 15 GHz frequency sweep range is obtained, the mismatch between the locking area and the free running slave laser frequency is the main constraining factors for the further improvement of the tuning range. A larger tuning range might be achieved using more accurate current compensation of the slave laser by programmable digital-to-analog converter to keep the slave laser follow the target sideband closely or using high injection ratio by an additional bandpass filter with sharp transition to suppress the 1st order sideband and a semiconductor optical amplifier to obtain higher-order sideband with sufficient injection power.

The slave laser’s power variation versus its swept frequency is also important to the frequency swept laser source. The drive current and the output power of the slave laser are monitored simultaneously by an oscilloscope (OSC) during the 15 GHz frequency chirp status and the results are shown in Fig. 6. The black curve is the drive current of the slave laser with external compensation during the frequency sweeping, and its variation tendency is consistent with the feedback controlling voltage to the VCO in our current driven control unit. The lower blue curve is the output power variation of the slave laser, and the output power fluctuates within 3.2% over the whole tuning range of 15 GHz. For comparison, we also measured the output power variation of the slave laser under the same current compensation but without optical sidebands injection, which is drawn as the upper blue curve and corresponds the lower blue cure with the same current. The difference between the two power variation curves is supposed to be caused by the optical injection. In that respect, the sweep range is also limited by the maximum operation current of the slave laser, so, temperature compensation could be introduced to get a wider frequency sweep range, but the sweeping speed could be limited.

 

Fig. 6 Experimentally measured slave laser’s drive current and output power versus swept frequency with 5th order sideband injection-locking.

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The settling response for the frequency switching is also measured. The measurement is performed by placing a fast photo detector (PD2, Thorlabs, DET01CFC, raise time 100 ps) at the output of the asymmetric Mach-Zehnder interferometer whose free spectrum range is 220 MHz, as shown in the right dashed box in Fig. 1. A continuous sawtooth ramp frequency sweeping RF signal with a repetition rate of 50 kHz is applied to the EOM. The amplitude of the frequency change is 10 MHz, and the instantaneous response of the slave laser can be caught by a high-speed OSC near the quadrature work point of the interferometer, so the instantaneous frequency changes can be calculated by using the measured slope of the interference fringe of the Mach-Zehnder interferometer. The experimental results of the VCO tune voltage and the frequency response signal are shown in Figs. 7(a) and 7(b) when the central frequency of the synthesizer is set at 5.5 GHz and 8.5 GHz respectively. The laser frequency has linear sweeping after several oscillations when the frequency changes suddenly. The settling time is measured to be 4 µs and 2.5 µs which is consistent with the 1MHz current source bandwidth and also the 1MHz PLL bandwidth, suggesting that the settling time is mainly limited by the current drive source and the PLL. A precise study of the current source and the PLL optimization will be the subject of future work, and we believe that a faster response might be obtained with advanced electrical circuits.

 

Fig. 7 Settling time response for the laser frequency switching when the frequency synthesizer is set at (a) 5.5GHz and (b) 8.5GHz. Frequency amplitude is 10 MHz.

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4. Conclusion

By using the combination of EOM driven high-order sideband-injection-locking and synchronous current compensation, the original 3 GHz tuning range and 0.5 THz/s tuning rate of a narrow linewidth laser is multiplied by 5 times to 15 GHz and 2.5THz/s respectively. The slave laser has a 3 dB-linewidth of 2.5 kHz which is the same to the master laser. From the experiments, there is a lot of room to expand the swept range through precise current compensation and higher-order sideband injection-locking. Furthermore, with modifications of the PLL in frequency synthesizer, the frequency noise and the settling time of the system can be improved. Finally, it is believed that arbitrary optical frequency sweeps can be achieved by tuning the frequency of the input RF signal arbitrarily. The proposed scheme has potential applications in phase coherent optical communication, synthetic-aperture imaging laser radar and other applications which require precise optical frequency controlling.