High Side-mode Suppression Ratio Research Articles

Injection-locked optoelectronic oscillator with high side-mode suppression ratio and low phase noise based on frequency-conversion delay matching

An injection-locked optoelectronic oscillator (OEO) based on frequency-conversion delay matching is proposed and experimentally demonstrated. There are two branches in the proposed OEO, where one branch is used to maintain the oscillation in the cavity, and the other one based on frequency conversion is used to generate a pure frequency-converted injection signal. Owing to the frequency-conversion-based injection locking effect, the side-mode suppression ratio of the proposed OEO can be greatly improved. In addition, the phase noise deterioration induced by the externally-injected signal can be effectively eliminated when the time delays of the two branches are well matched. In the experiment, a pure single-tone microwave signal at 9.99998 GHz is generated, whose side-mode suppression ratio and phase noise are measured to be 74.4 dB and −130 dBc/Hz@10 kHz, respectively.

Advances in wide-tuning and narrow-linewidth external-cavity diode lasers

The external-cavity diode laser is advantageous in terms of low noise, high side-mode suppression ratio, high temperature stability, simple structure, and low cost, which has been the preferred scheme to realize wide-tuning and narrow-linewidth characteristics. Thus, it has been widely used in optical communication, lidar, environmental monitoring, spectral analysis, optical coherence tomography, and other frontier fields. Herein, we introduce the technical scheme and review the development status of wide-tuning and narrow-linewidth external-cavity diode lasers in detail. Primarily, the structure, working principles, and performance characteristics of external-cavity diode lasers are deeply analyzed according to different structures. The structural characteristics, key technologies, optical performance and application fields of state-of-the-art studies in recent years are discussed. Finally, the challenges and potential development prospects of wide-tuning and narrow-linewidth external-cavity diode lasers are analyzed and discussed.

Experiment demonstration of high speed 1.3 µm grating assisted surface-emitting DFB lasers.

Surface emitting lasers are attractive light sources for silicon integrated photonic circuits. High speed direct operation is of great importance for these lasers in high capacity and low cost on-chip communication system. Here, we demonstrate a 1.3 µm surface emitting ridge-waveguide distributed feedback (DFB) laser with second order grating and λ/4 phase shift grating, which can achieve a 24 Gb/s operation over a wide temperature. The fabricated lasers can achieve low threshold current as 6.8 mA, and 12.5 mA at 20, and 70°C, respectively. Stable single mode operation has been observed with high side mode suppression ratio (SMSR) > 40 dB at all temperatures (20-70 °C). Meanwhile, the surface emitting optical power can reach 1.7 mW at high temperature as 70 °C. 3 dB bandwidth of small signal response is 21 GHz and 12 GHz at 20 °C and 70 °C respectively. The far-field divergence angle of surface emitting beam is 13.4°×20.2° of 10 µm length second order grating coupler. The proposed laser may have great advantages of single mode, high speed modulation and good temperature tolerance. In addition, compared with conventional DFB lasers, the surface emitting DFB laser has no additional manufacturing process, which is simple to fabricate and easy to integrate with silicon platform.

Widely tunable narrow linewidth laser source based on photonic molecule microcombs and optical injection locking.

We demonstrate a method to generate a widely and arbitrarily tunable laser source with very narrow linewidth. By seeding a coupled-cavity microcomb with a highly coherent single-frequency laser and using injection locking of a Fabry-Perot laser to select a single output comb tone, a high power, high side mode suppression ratio output wave is obtained. The system is demonstrated across 1530 -1585 nm with a linewidth below 8 kHz, having 5 dBm output power and sidemode suppression of at least 60 dB. Prospects of extending the performance are also discussed.

Research on Silicon-Substrate-Integrated Widely Tunable, Narrow Linewidth External Cavity Lasers

Widely tunable, narrow linewidth external cavity lasers on silicon substrates have many important applications, such as white-light interferometry, wavelength division multiplexing systems, coherent optical communication, and optical fiber sensor technology. Wide tuning range, high laser output power, single mode, stable spectral output, and high side-mode suppression ratio external cavity lasers have attracted much attention for their merits. In this paper, two main device-integrated structures for achieving widely tunable, narrow linewidth external cavity lasers on silicon substrates are reviewed and compared in detail, such as MRR-integrated structure and MRR-and-MZI-integrated structure of external cavity semiconductor lasers. Then, the chip-integrated structures are briefly introduced from the integration mode, such as monolithic integrated, heterogeneous integrated, and hybrid integrated. Results show that the silicon-substrate-integrated external cavity lasers are a potential way to realize a wide tuning range, high power, single mode, stable spectral output, and high side-mode suppression ratio laser output.

Microwave signal generation with discrete mode laser diode

Microwave signals with low phase noise can be broadly used in communication systems, satellite communications, and radar systems. Discrete mode laser diode (DM-LD) has the capacity to generate signals that meet the above requirements. Moreover, it has the following advantages such as stable operation over a wide temperature range, very high side mode suppression ratio, low sensitivity to optical feedback, and narrow linewidth. In this paper, we proposed and experimentally demonstrated DM-LD for generating the second harmonic signal in the self-starting microwave generation system. The results show that DM-LD effectively reduces the single sideband (SSB) phase noise of the fundamental signals and its second harmonic signals better than the same designed experimental system using a distributed feedback LD, which reduces the SSB noise of the fundamental signal/its second harmonic signal by over 16/22 dB, 18/22 dB, 13/19 dB at a frequency offset of 10 kHz, 100 kHz, and 1 MHz, respectively.

Self-Injection-Locked Optoelectronic Oscillator Based on Frequency Conversion Filtering

The multi-mode noise is an important issue for the optoelectronic oscillator (OEO) in generating a spectrally pure microwave. Especially in oscillators with low phase noise, increasing the cavity length to improve the phase noise performance will make the mode interval smaller, which is more likely to cause multi-mode noise. Here, we proposed a low phase noise and high side-mode suppression ratio self-injection-locked OEO based on frequency conversion filtering. We theoretically and experimentally studied the influence of the phase noise of the external microwave source on the proposed OEO and, as a comparison, the conventional injection-locked OEO. The results show that, compared with the traditional injection-locked OEO, the proposed OEO is less sensitive to the phase noise of the external microwave source. Under the same experimental conditions, the proposed OEO achieved the same side-mode suppression ratio and the phase noise is reduced by 30 dB at 10 kHz offset from the 10-GHz carrier compared with the conventional injection-locked OEO.

High SMSR and widely tunable multi-wavelength erbium doped fiber laser based on cascaded filters

In this paper, a widely tunable multi-wavelength erbium doped fiber laser (EDFL) with high side mode suppression ratio (SMSR) is proposed and experimentally demonstrated. With the cascaded structure of Sagnac loop filter (SLF) and a homemade twin core photonic crystal fiber (TCPCF), the proposed EDFL could achieve a wide tunable range without sacrificing the SMSR. By properly adjusting the polarization controller (PC) and the bending stage, switchable single-, dual-, and triple-wavelength lasing outputs can be obtained. The tuning range can reach up to 23 nm and the highest SMSR is 57 dB for single-wavelength lasing. Moreover, dual-wavelength lasing outputs with the tuning range of 19 nm and SMSR higher than 40 dB can also be obtained. The proposed widely tunable EDFL with high SMSR has great significance for many applications such as optical fiber communication, distributed fiber optic sensing and microwave generation.

Multi-channel wide range athermal operation of slotted surface grating lasers for athermal DWDM

Semiconductor lasers can be athermalised, which allows them to maintain their lasing wavelength with varying ambient temperature. Athermalisation can be achieved with careful control of injection currents into the laser diode. This allows the removal of a thermoelectric cooler and hence enables athermal wavelength division multiplexing. In this study, a high-order surface grating laser is operated athermally on eight, 100 GHz spaced channels from the ITU dense wavelength division multiplexing (DWDM) grid, achieving a wavelength stability of ±0.003nm/±0.4 GHz from 20 to 72–108°C depending on the channel. High side mode suppression ratios(>40 dB) and output powers(>+10 dBm) are also observed for a majority of the tuning ranges. Additionally, operation on six, 12.5 GHz spaced channels is also demonstrated with similar performance.

Tunable filter based on two concatenated symmetrical long period fiber gratings as Mach-Zehnder interferometer and its fiber lasing application

This work presents a highly stable switchable and tunable erbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) made with two symmetrical LPFGs separated by an SMF segment of 5 cm in length. The core and the cladding of the SMF central segment act like the MZI arms, whereas the LPFG1-SMF and SMF-LPFG2 splices act as optical couplers. The emission is switchable from one single emission at 1563.6 nm, a side mode suppression ratio (SMSR) of ∼55 dB, and linewidth of 0.15 nm, to a dual emission located at 1563.98 and 1573.89 nm, reaching a high SMSRs of 56 dB, and linewidths ∼0.10 nm. Moreover, the laser emission can be tuned from 1569 to 1572 nm applying torsion to the MZI from 0° to 330° obtaining a nearly linear sensitivity of 9.2 pm/° with low hysteresis. Additionally, the laser operation stability was tested over 60 min, obtaining the power and wavelength fluctuations lower than 0.086 dB and 0.028 nm respectively. To the best of our knowledge, this is the first time that symmetric LPFGs have been used as a torsion sensitive WSF. For its high wavelength stability and SMSR, the MZI can be a low-cost alternative WSF while the switchable and tunable laser can be useful as an optical source for spectroscopy and communications applications.

Femtosecond laser line-by-line inscription of apodized fiber Bragg gratings.

The reflection spectra of conventional fiber Bragg gratings (FBGs) with uniform index modulation profiles typically have strong sidelobes, which hamper the performance of FBG-based optical filters, fiber lasers, and sensors. Here, we propose and demonstrate a femtosecond laser line-by-line (LbL) scanning technique for fabricating apodized FBGs with suppressed sidelobes. This approach can flexibly achieve various apodized modulation profiles via precise control over the length and/or transverse position of each laser-inscribed index modification track. We theoretically and experimentally studied the influences of the apodization function on the side-mode suppression ratio (SMSR) in the fabricated apodized FBG, and the results show that a maximum SMSR of 20.6 dB was achieved in a Gaussian-apodized FBG. Subsequently, we used this method to fabricate various apodized FBGs, and the SMSRs in these FBGs were reduced effectively. Specifically, a dense-wavelength-division-multiplexed Gaussian-apodized FBG array with a wavelength interval of 1.50 nm was successfully fabricated, and the SMSR in such an array is 14 dB. Moreover, a Gaussian-apodized phase-shifted FBG and chirped FBG were also demonstrated with a high SMSR of 14 and 16 dB, respectively. Therefore, such an apodization method based on a modified femtosecond laser LbL scanning technique is an effective and flexible way to fabricate various FBGs with high SMSRs, which is promising to improve the performance of optical filters, fiber lasers, and sensors.

Distributed feedback lasers using surface gratings in Bragg waveguides.

This Letter presents, to the best of our knowledge, the first report of a narrow-linewidth ∼790-800nm edge-emitting semiconductor distributed feedback Bragg reflection waveguide diode laser (DFB2RL). The DFB2RLs were fabricated using a ridge waveguide structure with 5th order, surface-etched grating forming the wavelength selective element. Unbonded devices with a 500 µm cavity length exhibited continuous wave threshold currents in the region of 25 mA with an output power of 2.5 mW per (uncoated) facet at 100 mA drive current. The devices operated in a single longitudinal mode, with side-mode suppression ratio (SMSR) as high as 49 dB and linewidths as low as 207 kHz. Devices maintained single mode operation with high SMSR over a 9 nm wavelength range as the temperature was swept from 15°C to 50°C.

Switchable Ytterbium Fiber Laser Based on a Symmetrical Long-Period Fiber Grating

In this study, a switchable ytterbium-doped fiber laser based on symmetrical long-period fiber grating (LPFG) is presented. The LPFG is used as a wavelength-selective filter (WSF). Utilizing this WSF, the laser can emit one, two, or multiple stable output beams by adjusting the temperature on the LPFG from 20 °C to 600 °C. In addition, switching is also possible by adjusting the curvature of the LPFG from 0.399 to 0.709 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The symmetrical LPFG was fabricated by combining the cladding shaping with the thermal core expansion techniques by using a CO2-laser LZM-100 glass-processing system. The experimental results show a side-mode suppression ratio (SMSR) of 49 dB and a linewidth of 0.06 nm, within the range from 1038 to 1080 nm. In conclusion, the main achievement of this switchable laser is the high SMSR and stable output for high values of temperature, using a repeatable, simple, easy construction, and cost-effective laser tuning technique.

High-side mode suppression ratio with a high-stability external-cavity diode laser array at 976 nm in a wide temperature and current range

A high-side mode suppression ratio (SMSR) and high-stability external-cavity diode laser array employing optical feedback from a volume Bragg grating (VBG) with 15% diffraction efficiency at 976 nm was investigated. Based on the transfer matrix method and on the multimode rate equation, the theoretical model for an external-cavity diode laser at 976 nm was built for different temperature and current values. Both the simulated and the experimental results show that the temperature and the current range of such laser array with a front end reflectivity (Rf) of 0.5% are 10 °C and 15 A broader than that for an external cavity with Rf = 2%. As a result, the external-cavity diode laser array at 976 nm with a Rf value of 0.5% exhibits a high stability in the temperature range of 15–35 °C and from 15 to 50 A. Moreover, it shows an SMSR higher than 30 dB and a power larger than 33.9 W at 50A.

Broadband tunable single-longitudinal-mode erbium-doped fiber ring laser based on a microfiber knot resonator.

We propose and demonstrate a broadband tunable single-longitudinal-mode (SLM) erbium-doped fiber laser based on a microfiber knot resonator (MKR). The MKR is made from a double-ended fiber taper and employed for SLM filtering based on the Vernier effect. An unpumped erbium-doped fiber is used in the fiber laser cavity to suppress mode-hopping for a stabilized SLM laser operation. When combined with an optical fiber filter, widely tunable SLM laser generation is achieved. The proposed SLM laser can be tuned from 1545 to 1565 nm with a high side-mode suppression ratio of about 55 dB and a high stability.

Design and Simulation of Low-Threshold Miniaturized Single-Mode Nanowire Lasers Combined with a Photonic Crystal Microcavity and Asymmetric Distributed-Bragg-Reflector Mirrors.

A low-threshold miniaturized single-mode nanowire laser operating at telecommunication wavelengths was proposed and simulated. The device was constructed by combining a single InGaAs nanowire with a photonic crystal microcavity and asymmetric distributed-Bragg-reflector mirrors. The mode characteristics and threshold properties were calculated using the three-dimensional finite-different time-domain method. Due to the effective subwavelength confinement and strong optical feedback, provided by the photonic crystal microcavity, and distributed-Bragg-reflector mirrors, respectively, the confinement factor, end-facet reflectivity, and quality factor significantly improved. A lowest threshold of ~80 cm−1 and ultra-small cut-off radius of ~40 nm are obtained, reduced by 67%, and 70%, respectively, compared with a traditional nanowire laser. In addition, due to the photonic band gap effect, single-mode lasing is achieved with a high side-mode suppression ratio of >12 dB. By placing several identical nanowires in the photonic crystal with different lattice constants, an on-chip laser array is realized, which is promising in wavelength division multiplexing applications. This work may pave the way for the development of low-threshold miniaturized nanolasers and low-consumption high-density photonic integrated circuits.

1.7 μm Tm-doped continue-wave and pulse fibre laser using a modulated pump based on variable pulse generated mechanisms

A 1.7 μm Tm-doped continue-wave and pulse fiber laser based on variable pulse generated mechanisms is proposed and experimentally demonstrated. The modulated pump was used to generate pulses in the cavity by switching mechanisms easily (gain-switched, self-saturable absorber and active mode-locking via pump modulation (AMPM)). In the laser scheme, a 1.55 μm amplified optically modulated signal generator was used as a homemade pump, including a semiconductor laser, a pulse signal driver and an erbium-doped fiber amplifier (EDFA). A 3-m long thulium-doped fiber (TDF) was pumped to obtained gain which covered 1.7 µm band. A Fiber Bragg Grating (FBG) is employed as a spectrum filter. Using the proposed scheme, we achieved the laser with a central wavelength of 1727.92 nm. The laser had a high Side-Mode Suppression Ratio (SMSR) of 59 dB and a 3 dB linewidth of 0.18 nm. Additionally, we modulated the pump, and the pulse train with a repetition rate from 10 kHz to100 kHz based on gain-switched mechanism was obtained. Aim to increase the repetition rate, the generation of self-mode-locked pulses with a repetition rate of 11.39 MHz was observed associating with a self-saturable absorber using a CW pump. Moreover, We adjusted the modulated frequency of pump to match with the free spectral range of the cavity, the pulse train with repetition rate of 11.39 MHz was obtained with higher signal power based on AMPM, and the serval order harmonic mode-locking states with repetition rate from 22.78 MHz to 45.56 MHz were demonstrated with the appropriate pump frequency.

Single-mode surface-emitting DFB lasers with a large-area oxidized aperture based on the surface grating.

We propose and experimentally demonstrate an 850 nm single-mode surface-emitting distributed feedback laser based on the surface grating. The laser composes a second-order grating section sandwiched by two first-order grating sections. The second-order grating provides not only surface emission, but also phase shift to remove the emission degeneracy. A high side-mode suppression ratio of 47 dB has been achieved. By employing a large-area rectangle-shaped oxidized aperture, a low threshold current of 1.8 mA and a low differential resistance of 59 Ω are successfully achieved in a horizontal-cavity surface-emitting laser.

Phase-stabilized all-fiber-based mode-filtering technique for generating a gigahertz frequency comb.

An all-fiber-based mode-filtering technique is developed for generating a gigahertz-repetition-rate fiber-based frequency comb with a multiplication factor of 21. A high side-mode suppression ratio of approximately 65 dB is achieved by introducing a thermally diffused expanded core fiber between the dispersion compensating fiber and single-mode fiber to reduce splice loss. The fiber cavity length is also stabilized such that the resonance frequency is locked to the comb mode by applying the Pound-Drever-Hall stabilization technique. The proposed stabilized all-fiber-based mode-filtering technique is expected to be an attractive choice for a variety of applications that require a high-repetition-rate frequency comb.

InP-Based Surface-Emitting Distributed Feedback Lasers Operating at 2004 nm

We demonstrate InP-based buried grating coupled surface-emitting distributed feedback (DFB) lasers designed to operate at a wavelength of 2004 nm. The laser structure consists of three InGaAsSb/InGaAs QWs, with a 5 $\mu \text{m}$ wide double-channel ridge waveguide. A second-order semiconductor/semiconductor grating is used for in-plane feedback and vertical out-coupling. The single longitudinal mode emission wavelength of the fabricated laser can be adjusted from 2002.7 to 2006 nm without any mode hopping. High side-mode suppression ratio (SMSR) of at least 35 dB is achieved under all injection currents and temperature conditions. The edge output power reaches 19 mW, measured in continuous-wave (CW) mode at 10 °C. Simultaneously, the output power of surface emission reaches 8 mW.