Widely Tunable Two-Section Directly Modulated DBR Lasers for TWDM-PON System

Wavelength tunable and directly modulated distributed Bragg reflector(DBR)lasers with butt-joint technology are designed,fabricated and characterized.The DBR laser consists of a gain section and a DBR section.To increase the electrical isolation between the gain section and the DBR section,parts of a p-doped material in the isolation region are etched off selectively.Over 2 kΩ isolation resistance is realized ultimately without the need of ion implantation,which simplifies the fabrication process.The laser exhibits high speed modulation with a large tunable range.The 3 dB direct modulation bandwidth of the device is over 8 GHz in a 12 nm tunable range.This widely tunable DBR laser with the simple structure is promising as a colorless light source for the next-generation time and wavelength division multiplexed passive optical network(TWDM-PON)systems.     

Self-aligned Coupled Waveguide Distributed Bragg Reflector Lasers

A novel self-aligned coupled waveguide (SACW) multi-quantum-well (MQW) distributed Bragg reflector (DBR) laser is proposed and demonstrated for the first time. By selectively removing the MQW layer and leaving the low SCH/SACW layer the Bragg grating is partially formed on this layer. By optimizing the thickness of the low SCH/SACW layer, a-80% coupling efficiency between the MQW gain region and the passive region are obtained. The typical threshold current of the SACW DBR laser is 39 mA, the slope efficiency can reach to 0.2 mW/mA and the output power is more than 20 mW with a more than 30dB side mode suppression ratio.     

Dual-wavelength distributed Bragg reflector semiconductor laser based on a composite resonant cavity

We report a monolithic integrated dual-wavelength laser diode based on a distributed Bragg reflector (DBR) composite resonant cavity. The device consists of three sections, a DBR grating section, a passive phase section, and an active gain section. The gain section facet is cleaved to work as a laser cavity mirror. The other laser mirror is the DBR grating, which also functions as a wavelength filter and can control the number of wavelengths involved in the laser action. The reflection bandwidth of the DBR grating is fabricated to have an appropriate value to make the device work at the dual-wavelength lasing state. We adopt the quantum well intermixing (QWI) technique to provide low-absorption loss grating and passive phase section in the fabrication process. By tuning the injection currents on the DBR and the gain sections, the device can generate 0.596 nm-spaced dual-wavelength lasing at room temperature.     

Single-frequency,single-polarization ytterbium-doped fiber laser by self-injection locking

We demonstrated a stable single-frequency,single-polarization operation of ytterbium-doped fiber laser. As a novel practical method to realize single-polarization operation of fiber distributed Bragg reflector (DBR) laser,we proposed self-injection locking (SIL) with an active fiber ring feedback cavity.The laser has high output power exceeding 15 mW,wavelength of 1053.20 nm,and side-mode suppression ratio greater than 60 dB. The SIL fiber laser shows the improvements in output power and side-mode suppression compared with the fiber DBR laser.No mode-hopping is observed within 2 hours.     

Dual-Wavelength Holmium-Doped Fiber Laser Pumped by Thulium-Ytterbium Co-Doped Fiber Laser

We present an all-fiber dual-wavelength holmium-doped fiber laser operating in 2 μm region using a newly developed holmium-doped fiber(HDF) as a gain medium.The proposed fiber laser is constructed by using a hybrid gain medium,i.e.,a thulium-ytterbium co-doped fiber(TYDF) and an HDF in conjunction with a simple half-opened linear cavity,which is formed by a broadband mirror and an output coupler reflector.Without the HDF,the TYDF laser operates at wavelengths of 1991 and 1999 nm with a signal-to-noise ratio of more than 34 dB and the slope efficiency of 26.16%.With the HDF,dual-wavelength output lines are obtained at 2075 and 2083 nm with signal-to-noise ratios of more than 17 dB,3dB bandwidth of less than 0.2 nm and the power difference between the two peaks of less than 1 dB at the TYDF laser pump power of 320 mW.     

Read—out of a read—only super—resolution optical disc with a Si mask

A novel read-only super-resolution optical disc structure (substrate/mask layer/dielectric layer) is proposed in this paper. By using a Si thin film as the mask layer, the recording pits with a diameter 380nm and a depth 50nm are read out on the dynamic measuring equipment; the laser wavelength α is 632.8nm and the numerical aperture is 0.40. In the course of reproduction, the laser power is 5mW and the rotation velocity of the disc is 4m·s^-1. The optimum thickness of the Si thin film is 18nm and the signal-to-noise ratio is 32dB.     

Achieving single-lobed far-field patterns of broad area laser diode with external cavity feedback

We demonstrate a technique for single transverse mode operation of high-power broad area laser diode (BAL). In the experiment, the HR mirror is used as an external cavity mirror and the grating is used as a wavelength selective component. By tiling the HR mirror and the grating, the number of transverse modes oscillating in the cavity can be limited and the spectral bandwidth of the laser diode can be reduced. A single-lobed near diffraction-limited laser beam with the beam divergence (FWHM) of 0.43°, the spectral line-width of 0.7 nm and the output power of 350 mW are obtained. With the feedback, the power density of the output laser beam is increased 6 times in comparison with the free running.     

A novel dual-wavelength fiber Bragg grating and its application in fiber ring laser

A novel method for fabricating dual-wavelength fiber Bragg gratings (FBGs) by using one phase mask is developed. The method is based on a double-exposure technique. Our technique lends itself to writing gratings with controllable reflectivity and separation of two Bragg wavelengths. A grating with two equal transmission peaks of 20.25 dB is obtained by this method and the separation of the two Bragg wavelengths is about 0.8 nm. With the grating, we demonstrate a dual-wavelength erbium-doped fiber ring laser whose interval of the two peaks is 0.8 nm. The laser's peak powers can get 3.1 mW above and have a good stability.     

A strain-induced birefringent double-clad fiber Bragg grating

A strain-induced birefringence double-clad (DC) fiber Bragg grating (FBG) is proposed and demonstrated.The grating is fabricated in the core of rectangular inner cladding double clad fiber by using phase mask method. By applying lateral strain on the grating, the birefringence is induced. In order to detect the birefringent effect of the grating, we use it as the output mirror of a laser. When lateral strain is applied,the grating becomes birefringent. Therefore, one reflection peak of double-clad fiber Bragg grating becomes two peaks and the laser also lases in two wavelengths. The wavelength spacing of the laser can be tuned from 0 to 0.8 nm. The absolute wavelengths for the two polarizations can be tuned 1.2 and 2.0 nm,respectively.     

Low noise gain-clamped L-band erbium-doped fiber amplifier by utilizing fiber Bragg grating

A novel gain-clamped long wavelength band (L-band) erbium-doped fiber amplifier (EDFA) is proposed and experimented by using a fiber Bragg grating (FBG) at the input end of the amplifier. This design provides a good gain clamping and decreases noise effectively. It uses two sections of erbium-doped fiber (EDF) pumped by a 1480-nm laser diode (LD) for higher efficiency and lower noise figure (NF). The gain is clamped at 23 dB with a variation of 0.5 dB from input signal power of -30 to -8 dBm for 1589 nm and NF below 5 dB is obtained. At the longer wavelength in L-band higher gain is also obtained and the gain is clamped at 16 dB for 1614 nm effectively. Because the FBG injects a portion of backward amplified spontaneous emission (ASE) back into the system, the gain enhances 5 dB with inputting small signal.     

Transverse-mode controlling of a large-mode-area multimode fiber laser

Coiling technique is used to control the transverse mode of a large-mode-area (LMA) multimode fiber laser. By winding the fiber to a coil with different radius, high-order modes of a multimode fiber laser are suppressed one by one and finally 15.4-W single-transverse-mode output is achieved when the coil radius is 20 mm. It is found that as the coil radius decreases, the beam quality of a multimode fiber laser gets better but the slope efficiency drops for higher-order modes are discriminated. During the experiment, as the coil radius of multimode fiber changes, output characteristic of the laser has been measured. Meanwhile,the mode loss of different modes is calculated theoretically. It is proved that the experimental measured results fit well with the theoretically calculated results.     

Transverse-mode controlling of a large-mode-area multimode fiber laser

Coiling technique is used to control the transverse mode of a large-mode-area(LMA)multimode fiber laser.By winding the fiber to a coil with different radius,high-order modes of a multimode fiber laser are suppressed one by one and finally 15.4-W single-transverse-mode output is achieved when the coil radius is 20 mm.It is found that as the coil radius decreases,the beam quality of a multimode fiber laser gets better but the slope efficiency drops for higher-order modes are discriminated.During the experiment,as the coil radius of multimode fiber changes,output characteristic of the laser has been measured.Meanwhile, the mode loss of different modes is calculated theoretically.It is proved that the experimental measured results fit well with the theoretically calculated results.     

All-fiber ring-cavity based frequency swept laser source for frequency domain OCT

<正>We develop a high-speed tunable,quasi-continuous-wave laser source for frequency domain(FD) optical coherence tomography(OCT).The laser resonance is realized within a unidirectional all-fiber ring cavity consisting of a fiber coupler,two fiber isolators,a semiconductor optical amplifier(SOA),and a fiber FabryPerot tunable filter(FFP-TF) for frequency tuning.Light output from the coupler is further amplified and spectral shaped by a booster SOA terminated at both ends with two isolators.The developed laser source provides up to 8000 sweeps per second over a full-width wavelength tuning range of 120 nm at center wavelength of 1320 nm with an average power of 9 mW,yielding an axial resolution of 13.6μm in air and a maximum sensitivity of about 112 dB for OCT imaging.The instantaneous linewidth is about 0.08 nm,enabling OCT imaging over an axial range of 3.4 mm in air.For optimization consideration based on this custom-built swept laser,experimental study on imaging quality relevant parameters of the swept laser with sine and ramp driving waveforms to the FFP-TF is conducted,and investigation of the swept laser on the cavity length is done.Implementing the laser source in our established swept source based OCT(SS-OCT) system,real-time structural imaging of biological tissue is demonstrated.     

Generation of synchronized femtosecond and picosecond laser pulses in a two-beam-pumped Ti:sapphire laser

<正>Two operating modes,independent self-mode-locking and cross-mode-locking,are presented in a two-beampumped double-cavity dual-wavelength femtosecond Ti:sapphire laser.Synchronization of femtosecond and picosecond laser pulses is achieved by properly adjusting the cavity length matching and distributing the pump laser powers in the two laser cavities,and moreover,a timing jitter of 517 fs between femtosecond and picosecond pulses is obtained,with wavelength tuning ranges around 36 and 22 nm in the femtosecond and picosecond cavities,respectively.     

Mid-IR Dual-Wavelength Difference Frequency Generation Using Fiber Lasers as Pump and Signal Light Sources

A continuous-wave mid-IR difference frequency laser source, which respectively uses an ytterbium-doped fiber laser as the pump source and a multiwavelength erbium-doped fiber laser cascaded with an erbium-doped fiber amplifier as the signal source, is demonstrated. Our experimental results show that two stable mid-IR radiation lines with a spacing of about 5.4nm may be simultaneously emitted by a suitable setting the pump and signal polarization orientations. The number of the mid-IR radiation lines is limited by the quasi-phase-matching acceptance bandwidth. By changing the PPMgLN temperature the two mid-IR radiation lines may be synchronously tuned in the mid-IR range between 3295 and 3356.3nm.     

An all-fiber type Er~(3+)/Yb~(3+) co-doped fiber laser

In this paper, a distributed Bragg reflection (DBR) type Er~(3+)/Yb~(3+) co-doped fiber laser of high output power and high slope efficiency was developed. Its gain medium was a 4.45-m-long Er3+/Yb3+ co-doped fiber. When it was pumped by a 1064-nm Nd:YAG, the linewidth of output laser was measured as 0.072 nm by 3 dB and 0.192 nm by 25 dB at 1552.08 nm. The maximum output power was measured as 69 mW. Its power stability was < 5%, side mode suppression ratio was 59 dB, and the output wavelength stability was ±0.01 nm. The laser had a threshold of 12 mW and a slope efficiency of 22%.     

Novel method for measurement of effective cavity length of DBR fiber

<正>A novel method to measure the effective cavity length of distributed Bragg reflector(DBR) fiber laser with fiber Bragg grating(FBG) Fabry-Perot cavity as the resonator is proposed.The effective cavity length of DBR fiber laser is accurately measured by measuring the frequency spacing of two adjacent longitudinal modes in the cavity without the precise physical length of FBGs.The measuring accuracy as high as 10~(-5) mm can be obtained by setting the resolution bandwidth of the spectrum analyzer.     

Holographic Laser as a Source of Continuous Variable Entanglement

We study the generation and evolution of continuous-variable entanglement in a holographic laser. The two-level atomic medium is trapped in a ring cavity, and couples with two counter-propagating modes. By simulating the dynamics of the system, our numerical results show that the two-mode continuous variable （CV） entanglement can be realized in the present system even in the presence of cavity loss. This investigation provides a research clue for realizing CV entanglement in a two-level atomic medium, which is simpler than the previous works.     

Coherence transfer from 1064 nm to 578 nm using an optically referenced frequency comb

A laser at 578 nm is phase-locked to an optical frequency comb(OFC) which is optically referenced to a subhertzlinewidth laser at 1064 nm. Coherence is transferred from 1064 nm to 578 nm via the OFC. By comparing with a cavitystabilized laser at 578 nm, the absolute linewidth of 1.1 Hz and the fractional frequency instability of 1.3 × 10-15 at an averaging time of 1 s for each laser at 578 nm have been determined, which is limited by the performance of the reference laser for the OFC.     

Stable High Power and High Beam Quality Diode-Side-Pumped Continuous-Wave Intracavity Frequency-Doubled Nd:YAG Laser

We report a stable high power and high beam quality diode-side-pumped cw green laser from intracavity frequency-doubled Nd： YAG laser with KTP. By using a L-shaped concave-convex resonator, designed with two Nd：YAG rods birefringence compensation, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously. The green laser delivers a maximum 532nm output power of 23.2 W. Under 532nm output power of 20.9 W, the beam quality factor is measured to be 4.1, and the fluctuation of the output power is less than 1.4% in an hour.