kHz Linewidth Research Articles

Optical pulse generation in single section InAs/GaAs quantum dot edge emitting lasers under continuous wave operation

Observation of sub-picosecond pulses from a single section Fabry Perot InAs/GaAs edge emitting quantum dot (QD) based laser at 1.3 μm under continuous wave operation is reported. After group delay dispersion compensation, pulse durations as short as 770 fs in a 45 GHz repetition rate device have been measured, with 1.9 W of peak power and a narrow radio frequency spectrum of only a few kHz linewidth. The experiments show evidence of an unexplored mode locking regime in the InAs/GaAs quantum dot material system, which still needs theoretical modelling and further analysis.

Narrow linewidth single frequency microfiber laser

A compact 2 kHz linewidth single frequency microfiber ring laser is demonstrated. Microfiber, with a diameter of 1.88 μm, which is drawn from an Er(3+)/Yb(3+) co-doped phosphate glass fiber, serves as the gain medium. By using this microfiber, a double-knot resonator with a total length of 1.75 mm is constructed. Based on this resonator, a narrow linewidth single frequency laser with output power higher than 0.95 μW is obtained at the wavelength of 1536.1 nm. The linewidth of this microfiber laser is as narrow as 2 kHz, and the side-mode-suppression ratio is higher than 38 dB.

Experimental investigation of the equalization-enhanced phase noise in long haul 56 Gbaud DP-QPSK systems

We experimentally demonstrate the impact of equalization-enhanced phase noise (EEPN) on the performance of 56 Gbaud dual-polarization (DP) QPSK long haul transmission systems. Although EEPN adds additional noise to the received symbols, we show that this reduces the phase variance introduced by the LO laser, and therefore should be considered when designing the carrier phase recovery (CPR) algorithms and estimating system performance. Further, we experimentally demonstrate the performance degradation caused by EEPN when a LO laser with a large linewidth is used at the receiver. When using a 2.6 MHz linewidth distributed feedback (DFB) laser instead of a ~100 kHz linewidth external-cavity laser (ECL) as a LO, the transmission distance is reduced from 4160 km to 2640 km due to EEPN. We also confirm the reduction of the phase variance of the received symbols for longer transmission distances showing its impact on the CPR algorithm optimization when a DFB laser is used at the receiver. Finally, the relationship between the EEPN-induced penalty versus the signal baud rate and the LO laser linewidth is experimentally evaluated, and numerically validated by simulations.

Joint phase noise and frequency offset estimation and mitigation for optically coherent QAM based on adaptive multi-symbol delay detection (MSDD)

This paper extends our prior coherent MSDD Carrier Recovery system from QPSK to QAM operation and also characterizes for the first time the Carrier Frequency Offset (CFO) mitigation capabilities of the novel MSDD for QAM systems. We introduce and numerically investigate the performance of an improved MSDD carrier recovery system (differing from the one disclosed in our MSDD for QPSK prior paper), automatically adapting to the channel statistics for optimal phase-noise mitigation. Remarkably, we do not require a separate structure to estimate and mitigate CFO, but the same adaptive structure originally intended for phase noise mitigation is shown to also automatically provide frequency offset estimation and recovery functionality. The CFO capture range of our system is in principle infinite, whereas prior CFO mitigation systems have CFO capture ranges limited to a small a fraction of the baud-rate. When used for 16-QAM with coherent-grade lasers of 100 KHz linewidth, our MSDD system attains the best tradeoffs between performance and complexity, relative to other carrier recovery systems combining blind-phase-search with maximum likelihood detection. We also present additional MSDD phase-noise recovery system variants whereby substantially reduced complexity is traded off for slightly degraded performance. Our MSDD system is able to switch "on-the-fly" to various m-QAM constellation sizes, e.g. seamlessly transition between 16-QAM and QPSK, which may be useful for dynamically adaptive optical networks.

Linewidth of a quantum-cascade laser assessed from its frequency noise spectrum and impact of the current driver

We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ${\approx} 1~\mbox{nA/}\sqrt{}\mbox{Hz}$ allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ${\approx} 1.6~\mbox{MHz/}(\mbox{nA/}\sqrt{}\mbox{Hz})$ for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several $\mbox{nA/}\sqrt{}\mbox{Hz}$ , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only $350~\mbox{pA/}\sqrt{}\mbox{Hz}$ current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.

Ultrashort Pulse Generation at Quasi-40-GHz by Using a Two-Section Passively Mode-Locked InGaAsP-InP Tensile Strained Quantum-Well Laser

A 1.56 μm passively mode-locked laser diode with a two-section tensile strained multi-quantum-well structure is fabricated. Without any external pulse compression, a Lorentz pulse train with a pulse width of 1.03 ps and a repetition rate of 35.6 GHz is obtained, which is one of the best results that have been reported on similar devices. The optical pulse has a 300 kHz line width and a 50 dB peak over the noise floor in the photodetected radio-frequency electrical spectrum.

Semiconductor disk laser at 2.05 μm wavelength with <100 kHz linewidth at 1 W output power

We report on an optically pumped single-mode GaSb-based semiconductor disk laser (SDL) emitting at 2.05 μm at an output power of 960 mW (1100 mW) with a side-mode suppression of better than 30 dB (20 dB). A linewidth of 60 kHz (sampling time: 100 μs) was measured at an output power of 960 mW. This improvement in output power over previous reports, which was achieved via a careful design of the SDL cavity paying close attention to the spatial overlap between cavity mode and pump spot, makes this class of SDL particularly attractive for remote sensing and optical free-space communication.

40 GHz mode-beating with 8 Hz linewidth and 64 fs timing jitter from a synchronized mode-locked quantum-dash laser diode

Linewidth narrowing of the radio frequency beat-tones and the optical-modes is experimentally investigated in a ∼40 GHz quantum-dash mode-locked laser diode subject to optical injection of 10 GHz pulses. In comparison to the 75 kHz linewidth exhibited by the beat-tones in passive mode-locking conditions, a remarkable reduction to less than 8 Hz is achieved when the laser is under optical injection. From this beat-tone signal, an integrated root-mean-square timing jitter of 64 fs is calculated. In addition, a quadratic profile of the optical linewidth with the wavelength is observed in active locking, reaching a minimum of 1.7 MHz for the longitudinal modes around 1530 nm and progressively increasing to 37.4 MHz for modes at 1525 nm.

Highly coherent terahertz wave generation with a dual-frequency Brillouin fiber laser and a 155 μm photomixer

Thanks to a portable dual-frequency Brillouin fiber laser and a 1.55 μm photomixer, we report the generation of a highly coherent kilohertz level submillimeter wave emission. Low-cost telecommunications components are used to achieve very simple source architecture. The photomixer is composed of a unitravelling carrier photodiode integrated with an antenna. An emission at 316 GHz is observed and analyzed thanks to heterodyne detection with a signal-to-noise ratio >65 dB and a ~1 kHz linewidth. The phase noise of the proposed source has the same performance at 1.7 and 316 GHz. We show that this source has comparable or better phase noise compared to electrical oscillators and the tunability is much wider.

29.2 W, 20 kHz linewidth all fiber single-frequency MOPA laser

We demonstrate a high power all fiber single-frequency MOPA laser. The center wavelength of the single-frequency fiber laser seed is 1064 nm with a linewidth narrower than 20 kHz. A two-stage power amplification configuration is employed. A single-frequency fiber laser with an output power of 29.2 W is obtained, and the optical to optical efficiency is 78%.

Photonic generation of microwave signals by exploiting fiber birefringence effect in single‐longitudinal‐mode distributed Bragg reflector fiber lazer

AbstractA novel all‐optical microwave generation technique based on fiber birefringence effect in single‐longitudinal‐mode (SLM) distributed Bragg reflector (DBR) fiber lazer is presented. Owing to the birefringence‐induced mode splitting, the proposed lazer could provide a microwave signal at ∼1.72 GHz with a 3 dB bandwidth of ∼30 kHz. Moreover, another microwave signal at 10.96 GHz could also be generated as transverse pressure was applied onto the proposed lazer. Similar phenomenon was observed when another DBR lazer constructed with Er/Yb co‐doped fiber was used, and a microwave signal at 11.47 GHz with ∼33 kHz linewidth have also been achieved. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 535–540, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24961

Single-frequency phosphate glass fiber laser with 100-mW output power at 1535 nm and its polarization characteristics

We demonstrate a high power single-frequency fiber laser. By using phosphate glass fiber as its gain media, the laser can achieve high output power of 100 mW in centimeter-long linear cavity. Single-frequency single-polarization operation with <5 kHz linewidth and 65-dB signal-to-noise ratio (SNR) is realized by using external cavity polarization feedback technology.

Narrowband tunable photonic notch filter

We propose a technique for realization of a high-contrast, tunable, low-insertion-loss notch filter using polarization selectivity of whispering-gallery-mode resonators. We demonstrate a 10 MHz filter with 5.5 dB insertion loss and 45.5 dB of in-band rejection. The measured rejection value is limited by the finite (3 kHz) linewidth of our laser. We show that the filter can potentially have tunable bandwidth without significant rejection modification.

6W, 1 kHz linewidth, tunable continuous-wave near-infrared laser

A modified Coherent 899-21 titanium sapphire laser is injection locked to produce 6-6.5 W of single-frequency light at 852 nm. After closed-loop amplitude control and frequency stabilization to a high-finesse cavity, it delivers 4-4.5 W with < 1 kHz linewidth at the output of a single-mode fiber. The laser is tunable from about 700-1000 nm; up to 8 W should be possible at 750-810 nm.

Short pulse generation using a passively mode locked single InGaAsP/InP quantum well laser

We report on subpicosecond pulse generation using passively mode locked lasers (MLL) based on a low optical confinement single InGaAsP/InP quantum well active layer grown in one epitaxial step. Systematic investigation of the performances of two-section MLLs emitting at 1.54 microm evidenced pulse width of 860 fs at 21.31 GHz repetition rate, peak power of approximately 500 mW and a time-bandwith product of 0.57. A 30 kHz linewidth of the photodetected radio-frequency electrical spectrum is further demonstrated at 21 GHz which is, to our knowledge, the lowest value ever reported for a quantum well device.

Short pulse generation using a passively mode locked single InGaAsP/InP quantum well laser

We report on subpicosecond pulse generation using passively mode locked lasers (MLL) based on a low optical confinement single InGaAsP/InP quantum well active layer grown in one epitaxial step. Systematic investigation of the performances of two-section MLLs emitting at 1.54 µm evidenced pulse width of 860 fs at 21.31 GHz repetition rate, peak power of ~500 mW and a time-bandwith product of 0.57. A 30 kHz linewidth of the photodetected radio-frequency electrical spectrum is further demonstrated at 21 GHz which is, to our knowledge, the lowest value ever reported for a quantum well device.

Subpicosecond pulse generation at 134 GHz and low radiofrequency spectral linewidth in quantum dash-based Fabry-Perot lasers emitting at 1.5 µm

Passive modelocking in one-section monolithic semiconductor laser diodes based on a quantum dash active layer at very high repetition rate (&gt;40 GHz), in the 1.5 µm window, is demonstrated. 800 fs pulse generation, without any pulse compression scheme, at 134 GHz, is reported. A 50 kHz linewidth of the radiofrequency (RF) spectrum at 42 GHz is also demonstrated, the lowest value reported for any semiconductor passively modelocked laser.

Brillouin fibre laser enhanced by Raman amplification

A multiple-wavelength Brillouin fibre laser is demonstrated, with channel spacings of ∼10.5 GHz, using Raman amplification in dispersion-shifted optical fibre. More than 20 frequency lines with ∼2 kHz linewidths are obtained using a 14.5 km length of fibre and a 1.3 MHz linewidth Brillouin pump. Gain depletion due to amplified spontaneous emission limits the total number of cascaded Brillouin lines. At high Raman pump powers, the laser produces a pulse train at the repetition rate corresponding to the cavity round-trip time.

Subkilohertz absolute-frequency measurement of the 467-nm electric octupole transition in171Yb+

The {sup 2}S{sub 1/2}(F=0){yields}{sup 2}F{sub 7/2}(F=3,m{sub F}=0) transition at 467 nm in a single trapped, laser-cooled ion of {sup 171}Yb{sup +} has been measured to be f{sub Yb{sup +}}=642 121 496 771.26(23) kHz using a femtosecond laser frequency comb generator. The measurement is limited by measurement statistics and by the ac Stark shift, both related to the 4.5 kHz linewidth of the probe laser at the time. The systematic shifts of the transition, including ac Stark, second-order Zeeman, quadrupole, and blackbody shifts, have been evaluated.

극미세선폭 단일모드 FBG 고리구조 공동 DFB 레이저

본 논문에서는 고리구조 FBG공동과 DFB 레이저를 결합하여 kHz대의 극미세선폭 레이저광을 발생시키는 새로운 방법을 제안하였다. 외부 FBG와 DFB 레이저내의 Bragg격자로 이루어진 긴 공동내에 고리구조를 형성함으로써 레이저 선폭을 압축시키는 방법을 사용하였다. 손실을 최대화하면서 모드홉핑이 일어나지 않는 최대 광궤환량을 유지하기 위하여 광회전기와 가변광감쇄기를 사용하였다. 이 방법의 타당성을 검증하기 위하여 63km의 광섬유 지연선로를 사용한 자기헤테로다인 측정장치를 사용하여 레이저 선폭을 측정한 결과 자기헤테로다인 측정장치의 해상도 한계인 3kHz 이하의 선폭을 얻었다. 이는 2<TEX>$\times$</TEX><TEX>$10^{-8}$</TEX> nm의 선폭에 해당한다 A novel ultra-narrow linewidth DFB laser in the extended FBG ring cavity having kHz linewidth is presented. The method of linewidth compression was realized by introducing an extended fiber ring cavity into the cavity between the external FBG and the Bragg grating inside a DFB laser diode. Optimum optical feedback control was achieved by using an optical circulator and a variable optical attenuator to prevent mode hopping induced by spatial hole burning. To prove the validity of the proposed scheme, a self-heterodyne measurement set-up with fiber delay line of 63 km was utilized for the ultra-narrow linewidth measurement. A 3 dB linewidth of less than 3 kHz was demonstrated, which is resolution limited performance. This linewidth is equivalent to 3 dB linewidth of 2<TEX>$\times$</TEX>10<TEX>$^{-8}$</TEX> nm.