David S. Wu

Direct Selection and Amplification of Individual Narrowly Spaced Optical Comb Modes Via Injection Locking: Design and Characterization

Many applications of optical frequency combs (OFCs) require manipulation and amplification of individual comb modes, e.g., arbitrary waveform generation, terahertz generation and telecommunications. Extracting individual comb modes can be a challenging task for OFCs with narrow comb mode spacings (100 MHz to 10 GHz) due to the limitations of conventional optical filters. Optical injection locking can address this problem, but-due to the relatively large bandwidth (1 to 10 GHz) required for simple (i.e., without the need for additional feedback loops) and stable locking-can struggle when processing OFCs with sub-GHz comb mode spacings. Here, we present an approach to optical injection locking which incorporates a dither-free phase locked loop that allowed for long-term locking to OFCs with comb spacings below the high power injection locking bandwidth. As a result, we achieved robust injection locking directly to a sub-GHz OFC (250 MHz in our experiments). Optimization of the optical injection power is carried out using detailed phase noise characterization. We achieved an Allan deviation for the frequency variation of the slave laser with respect to the injected comb mode (1 s gate time) down to 9.7 × 10-17 and 4.4 × 10-19 at 1 s and 1000 s averaging times respectively, and a phase error variance of 0.02 rad2 (integration bandwidth of 100 Hz to 500 MHz).

Optical Fourier synthesis of high-repetition-rate pulses

The ultimate goal in the generation of optical signals is optical arbitrary waveform generation, which would allow the generation of wide-bandwidth optical signals with arbitrary amplitude and phase profiles (e.g., custom-shaped short optical pulses or advanced telecommunications signals). Here we investigate a new route toward this goal based upon the coherent combination of multiple signals generated at different wavelengths from different lasers. We show how to address the various challenges associated with this approach and demonstrate the generation of 100 GHz repetition rate waveforms by combining five semiconductor lasers phase locked, via injection locking, to a common optical frequency comb. Independent control of the optical power and phase of each laser enabled the generation of customized waveforms. Our technique be should readily scalable to a larger number of lasers, promising a flexible source of ultrastable, high-repetition rate, large-bandwidth (>1 THz), shot-noise-limited, high-power optical waveforms.

Homodyne OFDM with Optical Injection Locking for Carrier Recovery

Homodyne detection provides the simplest digital signal processing (DSP) solution to optical coherent detection and minimizes the receiver bandwidth requirements. These features make it promising for high spectrally efficient formats such as optical orthogonal frequency domain multiplexing (OFDM), which has a flat optical spectrum and which is thus inherently sensitive to high-frequency distortions, e.g., due to limited detector bandwidth. The key to homodyne detection is recovery of the carrier from the received signal all optically (as opposed to frequency offset compensation via DSP. Herein, we use optical injection locking (OIL) in conjunction with carrier tone-assisted OFDM to achieve this. In contrast to previous reports, we show that OIL carrier recovery with subsequent homodyne detection can operate without the need for any optical prefiltering. First, we evaluate the performance as a function of the carrier tone guardband bandwidth. Further, we improve the robustness of this technique using a slow-phase lock loop that compensates for drift in the lasers temperature/current control electronics. Using this improved setup, we compare our all-optical-carrier-recovered homodyne and the “traditional” DSP-assisted intradyne detection for the case of OFDM-16QAM signals. Finally, we compare the computing complexity necessary for both approaches and estimate the intradyne performance limitations due to the carrier-local oscillator frequency offset.

Homodyne OFDM using simple optical carrier recovery

We use optical injection locking for carrier recovery in RF-pilot aided OFDM. Any need for optical pre-filtering is eliminated and only narrow guard bands are required. Improved performance with respect to heterodyne detection is demonstrated.

Optical injection locking-based amplification in phase-coherent transfer of optical frequencies

We demonstrate the use of an optical injection phase locked loop (OIPLL) as a regenerative amplifier for optical frequency transfer applications. The optical injection locking provides high gain within a narrow bandwidth (<100  MHz) and is capable of preserving the fractional frequency stability of the incoming carrier to better than 10(-18) at 1000 s. The OIPLL was tested in the field as a mid-span amplifier for the transfer of an ultrastable optical carrier, stabilized to an optical frequency standard, over a 292 km long installed dark fiber link. The transferred frequency at the remote end reached a fractional frequency instability of less than 1×10(-19) at averaging time of 3200 s.

Selective amplification of frequency comb modes via optical injection locking of a semiconductor laser: influence of adjacent unlocked comb modes

Optical injection locking can be used to isolate and amplify individual comb modes from an optical frequency comb (OFC). However, it has been observed that for narrow spaced OFCs (e.g. 250 MHz), the adjacent comb modes are still present in the output of the locked laser. These residual modes experience some amplification relative to the injected signal, however the gain is significantly less than for the locked mode. We report the measurement of this sidemode amplification for a semiconductor laser injection locked to a 250 MHz spaced OFC. It was found that this amplification can be well suppressed by tuning the frequency difference between the free running laser and the OFC mode it was locked to. The sidemode amplification was then investigated numerically by solving the laser rate equations under optical injection. It was found that the main contribution to the sidemode amplification was due to phase modulation induced by the residual comb modes. The detuning dependent suppression occurs due to destructive interference between pairs of equidistant comb modes.

Robust optical injection locking to a 250 MHz frequency comb without narrow-band optical pre-filtering

A semiconductor laser was injection locked to a single optical frequency comb mode with a dither-free phase locked loop. The standard deviation was 0.014Hz over 24 hours with an Allan deviation of 1 × 10⁻¹⁷ at 10s averaging.

Wavelength conversion technique for optical frequency dissemination applications

We demonstrate coherent wavelength conversion capable of covering the entire C-band by modulating the incoming optical carrier with a compact Fabry-Perot cavity embedded phase modulator and by optical injection locking a semiconductor laser to a tone of the generated optical frequency comb. The phase noise of the converted optical carrier over 1 THz frequency interval is measured to be -40 dBc/Hz at 10 Hz offset and the frequency stability is better than 2 × 10(-17) level for averaging times >1000 s, making this technique a promising solution for comparisons of state-of-the-art optical clocks over complex fiber networks.

Phase Noise and Jitter Characterization of Pulses Generated by Optical Injection Locking to an Optical Frequency Comb

Optical pulses were generated by injection locking two semiconductor lasers to individual modes of a 250-MHz spaced optical comb. The phase noise was analyzed up to 500-MHz. Results are complemented by pulse timing jitter measurements.

Stability Characterization of an Optical Injection Phase Locked Loop for Optical Frequency Transfer Applications

We propose an optical injection phase locked loop (OIPLL) as a high-gain amplifier for precise frequency transfer via optical fibers. The suitability of this approach for international optical clock comparison is evaluated.