Monolithic Mode-locked Laser Diode Research Articles

Laser diode based THz-TDS system with 133 dB peak signal-to-noise ratio at 100 GHz

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful and versatile tool in various scientific fields. These include—among others—imaging, material characterization, and layer thickness measurements. While THz-TDS has achieved significant success in research environments, the high cost and bulky nature of most systems have hindered widespread commercialization of this technology. Two primary factors contributing to the size and cost of these systems are the laser and the optical delay unit (ODU). Consequently, our group has focused on developing THz-TDS systems based on compact monolithic mode-locked laser diodes (MLLDs). The ultra-high repetition rate (UHRR) of the MLLD has the added benefit that it allows us to utilize shorter ODUs, thereby reducing the overall cost and size of our systems. However, achieving the necessary precision in the ODU to acquire accurate terahertz time-domain signals remains a crucial aspect. To address this issue, we have developed and enhanced an interferometric extension for UHRR-THz-TDS systems. This extension is inexpensive, compact, and easy to incorporate. In this article, we present the system setup, the extension itself, and the algorithmic procedure for reconstructing the delay axis based on the interferometric reference signal. We evaluate a dataset comprising 10,000 signal traces and report a standard deviation of the measured terahertz phase at 1.6 THz as low as 3 mrad. Additionally, we demonstrate a remaining peak-to-peak jitter of only 20 fs and a record-high peak signal-to-noise ratio of 133 dB at 100 GHz after averaging. The method presented in this paper allows for simplified THz-TDS system builds, reducing bulk and cost. As a result, it further facilitates the transition of terahertz technologies from laboratory to field applications.

Ultra-High Repetition Rate Terahertz Time-Domain Spectroscopy for Micrometer Layer Thickness Measurement.

Terahertz time-domain spectroscopy systems driven by monolithic mode-locked laser diodes (MLLDs) exhibit bandwidths exceeding 1 THz and a peak dynamic range that can compete with other state-of-the-art systems. Their main difference compared to fiber-laser-driven systems is their ultra-high repetition rate of typically dozens of GHz. This makes them interesting for applications where the length of the terahertz path may not be precisely known and it enables the use of a very short and potentially fast optical delay unit. However, the phase accuracy of the system is limited by the accuracy with which the delay axes of subsequent measurements are synchronized. In this work, we utilize an all-fiber approach that uses the optical signal from the MLLD in a Mach–Zehnder interferometer to generate a reference signal that we use to synchronize the detected terahertz signals. We demonstrate transmission-mode thickness measurements of stacked layers of m thick low-density polyethylene (LDPE) films.

Wideband Beam Steering Concept for Terahertz Time-Domain Spectroscopy: Theoretical Considerations

Photonic true time delay beam steering on the transmitter side of terahertz time-domain spectroscopy (THz TDS) systems requires many wideband variable optical delay elements and an array of coherently driven emitters operating over a huge bandwidth. We propose driving the THz TDS system with a monolithic mode-locked laser diode (MLLD). This allows us to use integrated optical ring resonators (ORRs) whose periodic group delay spectra are aligned with the spectrum of the MLLD as variable optical delay elements. We show by simulation that a tuning range equal to one round-trip time of the MLLD is sufficient for beam steering to any elevation angle and that the loss introduced by the ORR is less than 0.1 dB. We find that the free spectral ranges (FSRs) of the ORR and the MLLD need to be matched to 0.01% so that the pulse is not significantly broadened by third-order dispersion. Furthermore, the MLLD needs to be frequency-stabilized to about 100 MHz to prevent significant phase errors in the terahertz signal. We compare different element distributions for the array and show that a distribution according to a Golomb ruler offers both reasonable directivity and no grating lobes from 50 GHz to 1 THz.

Algorithm-based continuous pulse duration tuning and performance control of a mode-locked laser diode

A control algorithm is presented that addresses the stability issues inherent to the operation of monolithic mode-locked laser diodes. It enables a continuous pulse duration tuning without any onset of Q-switching instabilities. A demonstration of the algorithm performance is presented for two radically different laser diode geometries and continuous pulse duration tuning between 0.5 ps to 2.2 ps and 1.2 ps to 10.2 ps is achieved. With practical applications in mind, this algorithm also facilitates control over performance parameters such as output power and wavelength during pulse duration tuning. The developed algorithm enables the user to harness the operational flexibility from such a laser with 'push-button' simplicity.

Variable-in, Variable-out Optical Clock Recovery With an Optically Injection-Locked and Regeneratively Actively Mode-Locked Laser Diode

A novel method of variable-in, variable-out optical clock recovery using a monolithic mode-locked laser diode (MLLD) is proposed for future advanced and adaptive photonic networks. This method is based on regenerative active mode locking of the MLLD simultaneously optically injection-locked with an external seeding light and enables variable-in, variable out optical clock recovery in terms of variable wavelengths of input data light and recovered optical clock. The proposed method was experimentally validated at 40-Gbps optical clock recovery experiments. Stable clock recovery was successfully demonstrated, retaining almost unchanged pulse characteristics of the recovered optical clock pulses including low timing jitter (<;0.3 ps) and low frequency chirping, even when the wavelengths of the input data and the output clock were widely changed by over 30 nm (1535 nm to >;1570 nm).

Dynamics and Spectra of Monolithic Mode-Locked Laser Diodes Under External Optical Feedback

In this paper, the effect of external optical reflection on the dynamic regimes and spectral properties of Fabry-Perot and distributed Bragg reflector monolithic mode-locked laser diodes is investigated numerically. The relation of the findings to experimental results is discussed, and optimizing the laser construction for reducing the feedback effects is also assessed.

Optical Noise Tolerance in a Polarization-, Wavelength-, and Filter-Free All-Optical Clock Recovery System Based on a Monolithic Mode-Locked Laser

We experimentally investigated optical noise tolerance of a polarization-, wavelength-, and filter-free all-optical clock recovery system based on a monolithic mode-locked laser diode. The results showed that for input signal degradation, the jitter of the recovered clock did not degrade to approximately 8 dB/nm of the optical SNR. We also compared the clock recovery performance as functions of the wavelength and the modulation format (on-off keying and phase-shift keying) of the input data signals. We revealed that no significant changes were measured in the clock recovery performance despite such changes in the parameters of the input data signals, indicating that noise-tolerable clock recovery is expected, retaining the polarization- and the filter-free operations, regardless of the wavelength and the modulation format of the input data signals, by using our new scheme.

Polarization-, Wavelength-, and Filter-Free All-Optical Clock Recovery in a Passively Mode-Locked Laser Diode With Orthogonally Pumped Polarization-Diversity Configuration

We proposed and demonstrated all-optical clock recovery system using a monolithic mode-locked laser diode (MLLD) that operated with less sensitivity to the polarization and the wavelength of the input data signals also with no bandpass filter to eliminate the input signal-components. The keys to this new technique are the MLLD integrated with a tensile-strained quantum-well saturable absorber and a new polarization-diversity setup by signal input orthogonally polarized to the lasing polarization of the MLLD. This approach was experimentally validated in the 40 Gbps clock recovery experiments. The results exhibited excellent performance of the clock recovery with low timing jitters (<0.3 ps) remaining small dependence on the wavelength and the polarization of the input data signals and input-signal suppression less than -30 dB with no use of the bandpass filter. We also succeeded in the stable clock recovery for the input of polarization-scrambled data signals.

InGaAs–GaAs Quantum-Dot Mode-Locked Laser Diodes: Optimization of the Laser Geometry for Subpicosecond Pulse Generation

The effect of cavity geometry on both pulsewidth and pulse output power is systematically investigated for a monolithic mode-locked quantum-dot laser diode. Subpicosecond pulse generation is demonstrated, and trends in pulsewidth and pulse output power are correlated to the ratio of the gain-to-absorber section lengths. A three-fold reduction in the pulsewidth, from 2.3 ps to 800 fs, is observed for a change in the gain-to-absorber section length ratio of 14 : 1 to 3 : 1. In addition, an associated five-fold increase in average power and 14-fold increase in peak power are also observed.

Low Noise Performance of Passively Mode-Locked 10-GHz Quantum-Dot Laser Diode

We present a detailed investigation of the radio-frequency (RF) linewidth and noise performance of a 10-GHz packaged monolithic two-section quantum-dot mode-locked laser diode. Record low 500-Hz RF linewidths are reported under passive mode-locking operation, and the single sideband phase noise spectra and associated timing jitter performance under both passive and hybrid mode-locking is investigated.

Suppression of Q-switching instabilities in broadened-waveguide monolithic mode-locked laser diodes

It is predicted theoretically that broadening the optical confinement layer in monolithic mode-locked semiconductor lasers may suppress Q-switching instability, by increasing the carrier transport time, and lead to emission of shorter, more stable optical pulses.

Polarization-Insensitive Clock Recovery Operation in a Monolithic Passively Mode-Locked Distributed-Bragg-Reflector Laser Integrated With a Tensile-Strained Multiple-Quantum-Well Saturable Absorber

A 40-GHz monolithic mode-locked laser diode integrated with a tensile-strained multiple-quantum-well saturable absorber was developed for the purpose of polarization-insensitive all-optical clock recovery in a single chip of a semiconductor device. Clock recovery with small polarization sensitivity less than 0.2 dB was successfully achieved. We also demonstrated a stable clock recovery from polarization-scrambled data signals.

Retiming and reshaping function of all-optical clock extraction at 160 Gb/s in monolithic mode-locked laser diode

Retiming and reshaping behaviors in an all-optical clock extraction operation at 160 Gb/s using a monolithic 160-GHz mode-locked laser diode was investigated in detail. The investigation of the basic clock extraction performance revealed that the clock extraction was stable against the input of data signals with a long zero sequence and with a poor extinction ratio. The clock extraction performance when the input data had random intensity and timing fluctuations was also discussed. We revealed that the generated clock pulse train absorbed well the intensity and timing fluctuations of the input data. Stable clock extraction remained even when the input data had 25% of the peak-intensity fluctuations. A timing jitter reduction from 0.85 to 0.32 ps was also achieved. The detailed investigation in the frequency-domain analysis of the jitter spectra revealed that the timing jitter reduction originated from the short-term stability of the slave 160-GHz mode-locked laser diode.

All-Optical 160-Gb/s Clock Extraction With a Mode-Locked Laser Diode Module

All-optical clock extraction at 160 Gb/s was successfully achieved using a monolithic colliding-pulse mode-locked laser diode module. Low excess timing jitter less than 0.1 ps was achieved at the injection power of data signal from -3.5 to 3 dBm without any degradation of the pulse characteristics of the output clock pulses.

Time-domain measurement of pulse-timing fluctuations in a mode-locked laser diode

Time-domain measurement techniques have been applied to the evaluation of pulse timing fluctuations in a monolithic mode-locked laser diode operating at 20 GHz. The time interval analysis and the time-domain demodulation techniques, both developed by the present author, are utilized for the evaluation of passively and hybrid mode-locked lasers, respectively. Phase noise power spectral densities have been measured from 2.5 to 34 MHz Fourier frequency with 300-dB dynamic range.

Generation and stabilization of ultrafast optical pulse trains with monolithic mode-locked laser diodes

In this paper, we report on the generation and the stabilization of ultrafast optical pulse trains exceeding 100 GHz from monolithic mode-locked laser diodes (MLLDs) combined with some new techniques such as subharmonic synchronous mode-locking (SSML) and repetition-frequency multiplication (RFM) method. Key subjects to increase the pulse repetition frequencies of the MLLDs such as fast absorption recovery and harmonic mode-locking operation are discussed. 500 GHz optical pulse generation from a short-cavity, graded-index separated confinement heterostructure MLLD and THz-rate pulse generation by harmonic mode-locking are reported. We also demonstrate the stabilization of a 160 GHz MLLD by the SSML with subharmonic-frequency optical pulse injection and reveal that the SSML is very promising as a stabilization technique of the ultrafast MLLD beyond the limitations by the electronic device speed. A method to accurately measure the timing jitter of such ultrafast optical pulse train, all-optical down converting using a nonlinear optical device, is also presented. We also mention another choice for ultrafast optical pulse generation using the MLLD combined with a dispersive medium such as an optical fiber. We demonstrate here the generations of stable 84–256 GHz optical pulse trains by the RFM method of the MLLD stabilized by the SSML.

Precise operation-frequency control of monolithic mode-locked laser diodes for high-speed optical communication and all-optical signal processing

We describe a method for pulse-repetition-frequency tuning of mode-locked laser diodes (MLLDs) monolithically integrated with a distributed Bragg reflector (DBR). The pulse-repetition frequency, i.e., the cavity-roundtrip time, is tuned through loss-induced change in the effective length of the DBR. The frequency-tuning range as large as the chip-to-chip frequency deviation caused by cavity-length fabrication variation of 10 μm has been confirmed experimentally, and the MLLDs operate at SDH (synchronous digital hierarchy) frequencies of 9.953, 19.906 and 39.813 GHz. Synchronization with an external system-clock through the hybrid mode-locking operation reduces the timing jitter of the optical pulses to less than 0.3 ps. As an optical pulse source for optical communication, error-free 20-Gbps transmission over 3000 km has been demonstrated, confirming that the MLLD properties satisfy the requirements for use in real systems. The novel application of MLLDs to all-optical clock extraction, one of the essential functions required in all-optical signal processing, has been demonstrated at the 40-GHz SDH frequency.

Monolithic and multi-GigaHertz mode-locked semiconductor lasers: Constructions, experiments, models and applications

Progress in technology, theory and applications of high-frequency mode-locked diode lasers is reviewed. We present a systematic overview of a wealth of experimental and theoretical results and approaches reported recently in the field of physics, technology and applications of monolithic mode-locked laser diodes.

Precise SDH frequency operation of monolithic modelocked laser diodes with frequency tuning function

Frequency tuning has been achieved in monolithic modelocked laser diodes through electrical control of the distributed Bragg reflector (DBR) effective length. A 0.5% frequency-tuning range has been obtained, which is wide enough to compensate for the frequency error that results from cavity length uncertainty when the usual cleaving process is used. Low-jitter 9.95328 GHz SDH frequency operation with a 6 ps transform-limited pulse output has been demonstrated.

Recent works on monolithic mode-locked semiconductor laser diodes

Recent works on monolithic mode-locked semiconductor laser diodes