Back-side-coated chirped mirrors with ultra-smooth broadband dispersion characteristics

We demonstrate a new technique for the design of chirped mirrors with extremely smooth dispersion characteristics over an extended ultra-broadband wavelength range. Our approach suppresses spectral dispersion oscillations, which can lead to unwanted strong spectral modulations and limit the bandwidth of mode-locked laser pulses. Dispersion oscillations are significantly reduced by coating the chirped mirror structure on the back side of a substrate, providing ideal impedance matching between coating and ambient medium. An anti-reflection coating may be added on the front side of the substrate, geometrically separated from the chirped mirror. The chirped mirror structure and the anti-reflection coating are non-interfering and can be independently designed and optimized. The separation of both coating sections provides a much better solution for the impedance-matching problems than previous approaches to chirped mirror design. We show by a theoretical analysis and numerical simulations that minimum dispersion oscillations are achieved if the index of the substrate is identical to the index of one of the coating materials and if double-chirping is used for the chirped mirror structure. Based on this analysis, we design a mirror that supports a bandwidth of 220 THz with group delay dispersion oscillations of about 2 fs² (rms), an order-of magnitude improvement compared to previous designs of similar bandwidth. In a first experimental demonstration of back-side-coated (BASIC) mirrors, we achieve nearly transform-limited and virtually unchirped pulses of 5.8 fs duration from a Kerr-lens mode-locked Ti:sapphire laser. BASIC mirrors are particularly suited for higher-order dispersion compensation schemes. They support the extremely broad spectra of few-cycle pulses and promise to provide clean pulse shapes in this regime. Received: 19 April 2000 / Published online: 6 September 2000     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

500 nm带宽啁啾镜对的设计与应用

设计了中心波长在825 nm、有效带宽在600 nm～1100 nm的超宽带啁啾镜对,色散补偿量目标值为-100 fs2。配对设计的啁啾镜对有效抑制了单个啁啾镜的色散振荡,啁啾镜对色散振荡幅度的最大值为50 fs2,用设计制作的啁啾镜对进行色散补偿,用频率分辨光学开关测量装置对脉冲进行测量。将10.3 fs的超短脉冲通过0.5 mm厚的LBO晶体,色散效应导致脉冲展宽,半峰值处的全宽度为39.6 fs,实验结果表明,经过啁啾镜对一次色散补偿后的脉冲形状,半峰值处得全宽度为11.6 fs;脉冲经过2次补偿后,脉冲被压缩到10.7 fs。     

Optical pulse compression and amplitude noise reduction using a non-linear optical loop mirror including a distributed Gires-Tournois etalon

We propose and analyse numerically a novel dispersion-imbalanced non-linear optical loop mirror (NOLM) scheme allowing simultaneous compression and amplitude noise reduction of chirped ultrashort-pulsed signals. An all-fibre distributed Gires-Tournois etalon (DGTE) made of two uniform fibre Bragg gratings is inserted asymmetrically in the loop, and is used both to dispersion imbalance the NOLM and to compress the pulses. The results show that the output pulses are accompanied by side lobes, which originate from the no-uniformity of the DGTE dispersion spectrum. We analyse the formation of these side lobes and show that, in the case of highly chirped input pulses, side lobes are strongly reduced by the NOLM architecture. In this case, the device operates well even when the signal optical spectrum extends over an entire free spectral range of the DGTE. We believe that this work will be useful for the design of all-optical regeneration schemes for highly chirped data streams in optical transmission systems.     

Compression of attosecond harmonic pulses by extreme-ultraviolet chirped mirrors

In the race toward attosecond pulses, for which high-order harmonics generated in rare gases are the best candidates, both the harmonic spectral range and the spectral phase have to be controlled. We demonstrate that multilayer extreme-ultraviolet chirped mirrors can be numerically optimized and designed to compensate for the intrinsic harmonic chirp that was recently discovered and that is responsible for temporal broadening of pulses. A simulation shows that an optimized mirror is capable of compressing the duration from approximately 260 to 90 as. This new technique is an interesting solution because of its ability to cover a wider spectral range than other technical devices that have already been proposed to overcome the chirp of high harmonics.     

Design of chirped Mo/Si multilayer mirror in the extreme ultraviolet region

The chirped Mo/Si multilayer mirror in the extreme ultraviolet region is designed to obtain sub-femtosecond pulses from high-order harmonics. Numerical simulations of temporal profile of the pulses are made for superposition of incident high-order harmonics and that of reflected ones by the chirped multilayer mirror.The normal incidence reflectivity and chirp in the wavelength range of 12.5 - 16.5 nm are 6.7% ±0.5%and-3617±171 as2, respectively. Simulation results indicate that the designed chirped multilayer mirror can be used for producing 104-as pulses.     

Effect of the spectral distribution of a chirped pulse on the interaction of the high-power laser radiation with matter

The interaction of high-power picosecond laser radiation with solid targets is experimentally studied for the first time at various spectral distributions of a chirped laser pulse. The interaction of the high-power laser radiation with the target is studied at four regimes of the experimental setup: (i) at a relatively high contrast (10³) in the picosecond (Δt ～ 25 ps) range, (ii) at a relatively low contrast (3 × 10¹) in the picosecond (Δt ～ 25 ps) range, (iii) with spectral distortions of the chirped pulse, and (iv) with a strongly modulated spectrum of the chirped pulse. The results obtained reveal a strong dependence of the atomic and nuclear processes in the laser picosecond plasma on the spectral distribution of the chirped laser pulse. The prospects for the application of the spectral interferometry of chirped pulses for the online control of the parameters of the high-power laser radiation are demonstrated.     

Wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating

A novel wideband tunable linear-cavity fiber laser source using strain-induced chirped fiber Bragg grating (FBG) and erbium–ytterbium co-doped fibers is proposed and demonstrated for the first time. The strain-induced chirped FBG, which acts as a partial-reflecting mirror, is achieved by bending a uniform FBG bonded at a slant onto the lateral surface of a simply supported beam. A single wavelength lasing with a maximum wavelength tuning range of 17 nm has been achieved experimentally.     

Multiwavelength Linear-Cavity Fiber Laser Source Using a Sagnac Interferemetric Filter and a Strain-Induced Chirped Fiber Grating

A novel multiwavelength linear-cavity fiber laser source using a strain-induced chirped fiber Bragg grating, a Sagnac interferemetric filter, and Erbium-Ytterbium codoped fibers is proposed and demonstrated for the first time. The Sagnac interferemetric filter, which acts as a full-reflecting mirror, consists of a polarization-maintaining directional coupler and two pieces of polarization-maintaining fibers. The strain-induced chirped FBG, which acts as a partial-reflecting mirror, is achieved by bending a uniform FBG bonded at a slant onto the lateral surface of a simply supported beam. By tuning the strain-induced chirped FBG, desirable lasing wavelength and number can be easily achieved.     

Spatially-uniform temporal recompression of intense femtosecond optical pulses

A specially designed telescope with defocusing lens and off-axis parabolic mirror, which is working as a nonlinear element and producing self-phase modulation, was implemented for intense (3.1?TW/cm²) Fourier Transform Limit femtosecond laser pulses with Gaussian beam profiles. The pulse spectrum was broadened quasi- homogeneously over the beam cross-section due to the change in the lens thickness compensating for the reduction of the beam intensity from its center to periphery. In experimental demonstrations a set of chirped mirrors allowed for the spectral phase correction to a final pulse compression of 20?fs from 40?fs.     

Tunable high energy giant chirped passively mode-locked Yb-doped fiber laser

A tunable, low-repetition rate, all-normal-dispersion Yb-doped fiber laser (YDFL) that is passively mode locked based on a phase shifted long period fiber grating (PS-LPFG) is demonstrated and proposed. The mode-locking mechanism of the laser is based on nonlinear polarization evolution (NPE). Using a PS-LPFG as the spectral filter in the laser cavity, the mode-locked output wavelength can be tuned continuously and smoothly over a spectral range of 10 nm, which is the first time implementation of a tunable giant chirped pulse with all-fiber format bandpass filter in YDFL. The maximum output pulse energy is 38.9 nJ at the repetition rate of 2.499 MHz.     

高色散镜的设计和制备

设计和制备了两种高色散镜,分别在780-870 nm波长范围内提供约-800 fs² 群延迟色散补偿(group-delay dispersion,GDD)和在1030-1050 nm的波长范围内提供约-2500 fs²的群延迟色散补偿.设计的高色散镜用双离子束溅射方法进行制备.从白光干涉仪的测试结果可以看出,得到的-800 fs² GDD高色散镜和设计符合得比较好;-2500 fs² GDD的高色散镜用在掺Yb光纤激光器中很好的抑制了脉冲展宽.这是制备得到的国产高色散镜及在光纤激光器中应用的首次报道. 关键词： 高色散镜 群延迟色散 色散补偿 Yb光纤激光器     

Coherent synthesis of ultra-broadband optical parametric amplifiers

We report on coherent synthesis of two ultra-broadband optical parametric amplifiers, each compressed by chirped mirror pairs, resulting in almost-octave-spanning (520-1000 nm) spectra supporting nearly single-cycle sub-4 fs pulse duration. Synthesized pulse timing is locked to less than 30 as by a balanced optical cross-correlator. The synthesized pulse is characterized by two-dimensional spectral interferometry and has a 3.8 fs duration.     

Tunable wavelength division multiplexing channel isolation filter based on dual chirped long-period fiber gratings

We propose and demonstrate a wavelength tunable wavelength division multiplexing channel isolation filter based on two concatenated chirped long-period fiber gratings (LPGs). An intergrating space (IGS), deliberately introduced between the two gratings, provides an extra phase difference between the core and cladding modes. Changing this phase by heating the IGS without affecting the gratings tunes the channels. A theoretical account of the filter action is also presented and the results are found to be in excellent agreement with the experiments. Unlike the filters based on normal concatenated chirped LPGs without an IGS, the current filter shows a linear tuning over an increased spectral range.     

Wide angular range operation of a dielectric multilayer film with a photonic-crystal-type defect

Light can tunnel through a high-reflectivity dielectric multilayer film when a photonic-crystal-type defect is introduced in the structure, which is useful for optical signal processing. We consider chirped structures with a defect in layer thickness for which high reflectivity is achieved over a broad wavelength range except within a narrow spectral window. The useful transmission window, while it shifts toward shorter wavelengths as the angle of incidence of the light beam is increased, does not, in general, survive; i.e., transmission disappears progressively. We show that wide angular range operation can, however, be achieved by a proper design of the chirped structure. Analytical expressions for the design parameters are derived on the basis of a semi-infinite photonic crystal model. Theoretical reflectance spectra of defect SiO2/TiO2 chirped multilayer films are presented and discussed in terms of the dispersion of the electromagnetic radiation modes of the finite photonic crystal. These devices offer a simple way to mechanically tune (through inclination of the film) the wavelength transmitted from a fixed white-light beam.     

Step-changed period chirped long-period fiber gratings fabricated by CO2 laser

A CO₂ laser and point-by-point method are used for fabricating step-changed period chirped long-period fiber gratings (LPFG). Several types of period chirped LPFGs have been demonstrated, such as, linearly chirp, peak-shape chirp, and cascaded linearly chirp. Unlike uniform LPFGs, the spectrum change such as multiple attenuation peaks, broader spectrum can be seen in these chirp gratings, and the spectral shape can be controlled by the grating period. Especially, the cascaded linearly chirped LPFGs performs a multi-peak as interference between the core mode and cladding mode, which can be used as multi-wavelength filters in fiber optic communication and fiber optic sensors. Also, a linear tuning range of 1.6 nm with -0.559 pm/με tuning rate is achieved in these types of devices by applying an axial strain.     

Brewster-angled chirped mirrors for broadband pulse compression without dispersion oscillations

We use Brewster-angled chirped mirrors for dispersion compensation of a noncollinear optical parametric amplifier. This novel mirror design virtually eliminates spurious surface reflections and resultant dispersion ripple. The absence of compression artifacts is demonstrated by the generation of clean 5.6 fs pulses, with what is believed to be an unprecedented low ripple-induced satellite content for a nonadaptive scheme. In addition, the 270 THz spectral coverage allows generation of widely tunable visible pulses of 8 to 15 fs duration.     

1.5-octave chirped mirror for pulse compression down to sub-3 fs

We demonstrate numerically and experimentally a chirped mirror with controlled reflectivity and dispersion of up to 1.5 octaves. A complementary pair of such mirrors has a reflectivity of 95% in the wavelength range 400–1200 nm with residual group delay dispersion ripples <100 fs² in all of this range. The mirror pair allows one to compensate a chirp of the corresponding spectrum (with a smooth phase), resulting in sub-3-fs pulses. PACS 42.65.Re; 42.79.Fm; 42.79.Wc     

Two-dimensional optical ring resonators based on radial Bragg resonance

A Bragg-reflection-based ring resonator is proposed and analyzed. Closed-form expressions for the field and dispersion curves for radial Bragg gratings and photonic bandgap crystals are derived. The required gratings exhibit a chirped period and a varying index profile. Small bending radii and strong control over the resonator dispersion are possible by the Bragg confinement. Large free spectral range and low radiation loss are predicted theoretically.     

Chirp-coefficient bisection iteration method for phase-intensity reconstruction of chirped pulses

A new method is proposed to systematically measure the phase-intensity information of chirped pulses that is based on the chirp-coefficient bisection iteration (CBI) concept. Through the CBI procedure with measured spectrum and temporal intensity profiles (or intensity autocorrelations), spectral and temporal amplitude–phase information can be rapidly retrieved. We experimentally verified that our method has high precision for nanosecond- and picosecond-level pulses and low precision for femtosecond-level pulses. Our proposed method does not require a sophisticated setup and has the advantage of accurate determination of temporal and spectral chirp coefficients with various orders. It also has lower cost, simple operation, in particular covers a wider measurement range than the main current methods. Moreover, the retrieved waveforms can reveal both the pulse shape and the actual intensity with spectral and temporal chirped coefficients of various orders, which can be directly used in various pulse propagation analyses such as chirped pulse amplification.