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Single-mode optical fibers have been widely used in optical communications, and effective fiber lasers have been designed on the basis of active fibers with linear or ring cavities. In fiber lasers, the distance between the cavity mirrors can reach 270 km (the maximum length is determined by the linear attenuation and nonlinear dephasing of the waves). In 2009, random lasing was found in a long telecommunications fiber without any cavity: the positive distributed feedback required for lasing is due to Rayleigh scattering of light, and distributed amplification is provided by stimulated Raman scattering. Such a laser can be classified into the group of so-called random lasers, actively studied recently; the fiber geometry and the weakness of Rayleigh scattering provide much better output characteristics compared to the other types of random lasers. The lasing efficiency and beam quality of this laser are comparable to those of fiber lasers with a conventional cavity. At the same time, it has a number of unique features (unlimited length, and mode-free spectrum etc.), providing new physical phenomena and new opportunities for applications in telecommunications and sensor systems. The paper presents a review of recent results of studies in this area.

The results of the design and study of femtosecond fiber lasers are reviewed. Various methods of mode-locking and generation regimes are considered. Special attention is paid to the regime of dissipative solitons in an all-fiber resonator with normal dispersion. The main results and analysis of the possibilities of energy scaling of femtosecond pulses are given.

A cladding-pumped ytterbium-doped fibre laser has been tuned to shorter emission wavelengths (from 1040 to 1017 nm). The laser output power obtained has been compared to calculation results. We have studied frequency doubling of the laser in a KTiOPO4 (KTP) crystal with type II phase matching in the XY plane and demonstrated wavelength tuning in the range 510 — 520 nm.

The second-harmonic generation (SHG) efficiency for cw Yb-doped fibre laser radiation, which is characterised by many longitudinal modes with random phases, is compared with the SHG efficiency for amplified single-frequency Nd : YAG laser radiation in ppLN and KTP crystals, characterised by the type-I and type-IIphase matching, respectively. It is shown that the conversion efficiency into the second harmonic in the multifrequency regime for both crystals is higher by a factor of about 1.6, a value close to the calculated enhancement (2 for the Gaussian mode statistics). This difference is explained by possible deviation of the statistics of the Yb-doped fibre laser radiation from Gaussian, which is confirmed by measurements of the laser temporal dynamics.

A brief review of recent results obtained with LD-pumped Raman fiber lasers (RFLs) is given. Direct pumping of gradient-index fibers by multimode LDs offers RFL operation at ∼980 and ∼950nm with efficiency >40%. Herewith, the quality of output beam is greatly improved as compared with that of LDs. Further development in direction of all-fiber design and higher efficiency is shown.

We report the first measurements of the longitudinal power distribution in a twin-core optical fibre at different input light polarisations. Experimental evidence is presented that, because of the difference in birefringence between the cores, the power in them depends on which core the beam is launched into. Experimental data are interpreted in terms of a modified polarisation model for mode coupling in twin-core fibres which takes into account the birefringence of the cores. In addition, we demonstrate for the first time the use of the polarisation properties of a twincore fibre for mode locking in a fibre laser.

The possibility of apodisation of fibre Bragg gratings (FBGs) recorded in the interference region of two Gaussian beams in the phase-mask scheme is considered. The FBG reflection spectra are numerically simulated for different values of recordingbeam parameters and the distance between the axes of interfering beams diffracted into different orders, which is varied by transverse displacement of the phase mask with respect to the optical fibre. Suppression of side lobes and smoothing out of the FBG spectrum with an increase in the transverse displacement of the phase mask is experimentally demonstrated. It is shown that this effect is caused by the equalisation of the mean induced refractive index in the FBG region.

We report the results of numerical studies of the impact of asymmetric femtosecond pulses focused in the bulk of the material on the femtosecond modification of fused silica. It is shown that such pulses lead to localisation of absorption in the process of femtosecond modification and to a decrease in the threshold energy of modification. It is found that the optimal asymmetry parameters for reaching the maximum plasma density in the focusing region depend on the pulse energy: at an initial energy of about 100 nJ, it is preferable to use pulses with positive TOD; however, when the energy is increased, it is preferable to use pulses with negative TOD. This is explained by differences in the dynamics of the processes of absorption of energy of a pulse propagating in the material.

Generation of highly-chirped dissipative solitons (HCDS) in normal-dispersion cavity is treated now as a prospective technique to increase energy of femtosecond pulses delivered by an oscillator. Based on recently developed analytical model describing stable HCDS solutions, we have realized experimentally the regime of highly-chirped pulses in an Yb-doped fiber laser with passive mode locking via nonlinear polarization evolution (NPE) and tested the model predictions. It has been shown that the generated pulses satisfy the condition of equality for nonlinearity and dispersion induced phase shifts characterizing dissipative solitons, and their chirp parameter is growing nearly linearly with increasing cavity length. This results in nearly linear increase of the pulse energy, but the linear scaling is valid only up to some limit after which the stability of the regime is worsen significantly. At the limit length of ~ 5 m the pulses with nJ energy level and several ps duration are generated being compressed to ~ 200 fs out of the cavity. The reason of the stability break is shown to be an excessive rotation of polarization ellipse in the cavity, so it is proposed to use polarization-maintaining fibers to increase scaling range in HCDS regime obtained by means of NPE mode locking.

We present the results of comparative numerical study of femtosecond laser inscription for fundamental and second harmonic of Yb-doped laser. We have found that second harmonic is more efficient in terms of amount of absorbed energy which leads to lower inscription threshold. Hence this regime is more attractive for applications in femtosecond laser microfabrication. We observed the different size of modified domain on initial pulse energy and different spectrum dynamics during the pulse propagation for fundamental and second harmonics.

This paper presents an experimental study of frequency doubling of a tunable ytterbium-doped fibre laser in KTP crystals phase-matched in the XY and YZ planes. In the XY plane, we obtained continuous tuning in the range 528 — 540 nm through intracavity frequency doubling. The second-harmonic power reached 450 mW for 18 W of multimode diode pump power, which was five times higher in comparison with single-pass frequency doubling. In a single-pass configuration in the YZ plane, we obtained a wide tuning range (527 — 551 nm) in the green spectral region and a second-harmonic power of ~10 mW. The tuning range was only limited by the mechanical performance of the fibre Bragg grating and can potentially be extended to the entire lasing range of the ytterbium-doped fibre laser.

Continuous-wave parametric oscillation in a polarisation-maintaining optical fibre has been achieved for the first time using polarisation phase matching. Up-conversion with a frequency shift of ~8.6 THz has been demonstrated experimentally. The single-pass optical power generated at 1017 nm in a 85-m-long Nufern PM980-XP fibre exceeded 40 mW. The conversion efficiency was 3.3%, which is an order of magnitude higher than that reported previously for cw parametric up-conversion in the 1-μm spectral region. We compare theoretical predictions with experimental data and analyse factors that may influence output power saturation with increasing signal and pump powers.

The random distributed feedback fiber laser operating via the stimulated Raman scattering and random distributed feedback based on the Rayleigh scattering is demonstrated in the 1.2 μm frequency band. The RDFB fiber laser generates at 1174 nm up to 2.4 W of output power with corresponding slope efficiency more than 30%. The output radiation has the spectral shape similar to the conventional Raman fiber lasers and spectral width less than 1.7 nm.

The intracavity doubling of a tunable Yb-doped fiber laser is studied. The phase effects are observed, analyzed and compensated. A smooth wavelength dependence of the second harmonic is obtained in the wavelength interval 540–550 nm. The second-harmonic output is greater than 300 mW at an LD pump power of 18 W.

The frequency doubling and tripling in a Q-switched all-fiber laser are studied. It is demonstrated that the main limitations on the efficiency of the harmonic generation are related to the random polarization that is nonuniform with respect to axes, the asymmetric pulse shape with a flat trailing edge, and the significant spectral broadening of the multimode radiation of the fiber master oscillator in the fiber amplifier. The methods to increase the efficiency are proposed. For an IR pulse energy of about 0.3 mJ, duration of about 40 ns, and repetition rate of 1 kHz, the second- and third-harmonic pulse energies are greater than 60 and about 10 μJ, respectively.

A statistical model describing the Rayleigh-scattering reflectograms of narrowband Gaussian pulses in optical fibres is constructed taking into account linear (the finite spectral linewidth because of frequency fluctuations, finite frequency band of the photodetector) and nonlinear (modulation instability) effects influencing the reflectograms' visibility. The model is compared with the experiment, demonstrating the adequacy of the theoretical description. The possibilities of obtaining optimal parameters, important for practical applications of the phase-sensitive reflectometer as a distributed fibreoptic intrusion sensor are discussed.

The possibility of apodization of fibre Bragg gratings (FBGs) recorded in the region of interference of the two parts of a Gaussian beam in a Lloyd interferometer is considered. The reflection spectra of FBGs are numerically simulated for different parameters of the recording beam and its displacement with respect to the dividing axis in the interferometer. Aconsiderable suppression of sidelobe resonances in the FBG spectrum during the displacement of the beam centre with respect to the dividing axis by half the beam radius is predicted and experimentally demonstrated. It is shown that this is caused by the equating of the mean value of the refractive index in the FBG region.

The instability of the propagation of narrowband 100-ns laser pulses in the transparency window of a single-mode fiber in the presence of continuous broadband noise is demonstrated. The modulation instability developed due to the nonlinearity and anomalous dispersion leads to a rapid decay of the narrowband component after passing through the fiber. The modulation-instability threshold is shown to be sensitive to the noise level in a spectral interval of about 100 GHz near a central wavelength of about 1.55 μm and is inversely proportional to the fiber length (varied from 1.6 to 21.0 km in the experiments). The effect is completely eliminated owing to the application of a dispersion-shifted fiber with normal dispersion.

Operation of Yb-doped fiber laser (YDFL) at short wavelengths (about 1030 nm) is known to be possible with relatively short active fibers. Frequency doubling of such a laser is studied. It is shown that, in contrast to conventional YDFLs, operating around 1080 nm, the short-wavelength YDFL does not exhibit significant spectral broadening induced by nonlinear effects. Thus, the generated spectrum remains narrower than the FBG reflection profile, which allows an almost twofold intracavity intensity enhancement. Consequently, the second harmonic generation in the KTP crystal placed inside the YDFL cavity appears more effective than the generation in the single-pass scheme by an order of magnitude. An output power of about 0.4 W at a wavelength of 515 nm is demonstrated.

A new direct method for the measurement of the longitudinal power distribution in multicore fibers using the luminescence of active ions is proposed. The method is employed in the study of the effect of curvature of twin-core fiber on the interaction of modes that propagate in different cores. The experimental distributions are in qualitative agreement with the model of mode interaction in curved fiber with allowance for the photoelastic effect. The results are used to determine the length of the mode interaction and the difference of the core refractive indices.

655 nm laser radiation with power of >60 mW is generated by frequency doubling of a broadband randomly-polarized 1.31-μm phosphosilicate Raman fiber laser (RFL). The red power appears to grow linearly with increasing RFL power up to 7 W at efficiency comparable with that for single-frequency lasers. It has been shown that multiple sum-frequency mixing processes involving different RFL modes provide the main contribution to the output, which is enhanced by 2 times due to the modes stochasticity.

We demonstrate a Raman-based all-fibre temperature sensor utilising a pulsed erbium fibre laser. The sensor is made of a standard single-mode telecom fibre, SMF-28, and includes a number of directional couplers as band-pass filters. The temperature profile along a 7-km fibreoptic line is measured with an accuracy of 2oC and a spatial resolution of 10 m. In data processing, we take into account the difference in attenuation between the spectral components of the backscatter signal.

We report a single-frequency 1-W fibre laser source emitting at 1093 nm, composed of a distributed-feedback ytterbium fibre laser and fibre-optic amplifier. The laser frequency was stabilised by side-locking to a transmission peak of a Fabry — Perot interferometer, and the residual frequency noise spectrum of the laser was measured. Our results indicate that the laser linewidth can be narrowed down below 1 kHz.

We demonstrate that, in the presence of noise, propagation of narrow-band 100-ns pulses through a 6-km long optical fibre sharply reduces their amplitude when a threshold power (~0.2 W) is exceeded. The effect is due to the development of modulation instability and is sensitive to the noise level in the spectral region ~100 GHz in width around the central frequency.

A two-mirror multiple-beam interferometer with a noninverted response function in reflection is used for the frequency selection and tuning in fiber lasers. The simplest version of the interferometer is based on a thin metal (Troitsky) film. Such a reflection interferometer corresponds to an external (with respect to a fiber) two-mirror cavity that consists of a spherical highly reflective (99.8%) mirror and a plane mirror represented by the thin metal film deposited on the fiber end face. Tuning to the selected frequency is performed due to a variation in the distance between the mirrors.

The tuning capabilities at the frequency doubling of the Yb-doped fiber laser using type-II second harmonic generation (SHG) in a KTP crystal in a walk-off compensated geometry are studied. Three configurations for different wavelengths are compared and the best one that provides a continuous tuning in the range 540–560 nm due to crystal tilting is chosen. The intracavity frequency-doubling scheme is applied to increase the efficiency.

PACS numbers: A recent demonstration of lasing in ultralong Raman fiber lasers has attracted the attention to their specific features. In this paper, we analyze modifications of the mode structure, shape, and width of the generated spectrum, fluctuations of the intensity and phase upon the lengthening of such a laser, and increasing its power. The mode structure is shown to be resolvable in an RF spectra up to 84-km long (with ∼1 kHz spacing), and the RF peak width appears proportional to the intracavity power. At the same time, the optical spectra broaden with the power according to the square-root law and take the specific shape with exponential wings. The model based on the wave kinetic equations has been developed that describes the observed effects. It is shown that ultralong Raman lasers exhibit a new laser regime through the generation of multiple (up to ∼108) cavity modes interacting via FWM processes that lead to stochastic (turbulent) behavior of their phases and amplitudes.

A tuning method of a fiber Bragg grating by the bending compression-tension is considered. With this method, a relative tuning range of the resonance wavelength of the grating of about 5% has been obtained. The changes in the shape of the spectrum and in the grating reflectivity during the tuning process are studied. The reflectivity of the fiber Bragg grating has been found to considerably increase with increasing degree of compression. It is shown that such an increase is caused by compression-induced changes in the refractive index modulation amplitude in the grating. The fiber Bragg grating developed has been applied for tuning the radiation wavelength of an ytterbium fiber laser. The tuning range of the laser has been found to be 45 nm around the central wavelength 1080 nm.

A single-frequency all-fiber laser source operating at 1093 nm with an output power of more than 1 W is developed. A master oscillator-power amplifier configuration that comprises a distributed-feedback fiber laser and an ytterbium-doped fiber amplifier is implemented. Some aspects of the stimulated Brillouin scattering specific to active fiber are discussed.

An interrogator of Bragg sensors based on a narrow-band tunable erbium laser is developed. The laser characteristics enable one to interrogate up to 45 temperature sensors in a range of 100°C with an error of no greater than 1°C.

The resonance wavelength of the fiber Bragg gratings (FBGs) is tuned using two methods. Tunable FBGs are used as the selecting elements in the cavities of tunable lasers. An ytterbium-doped fiber laser with a wavelength tuning range of 1063–1108 nm and an output power of 6 W, a Raman fiber laser with a wavelength tuning range of 1252–1303 nm and an output power of 3 W, and an erbium-doped fiber laser with a wavelength tuning range of 1530–1580 nm are realized, and their characteristics are studied.

A reflectometric approach is proposed for the polling and multiplexing of sensitive elements on fiber Bragg gratings (FBGs). The method is based on the power measurement of the radiation reflected by the FBG using a conventional fiber-optic time-domain reflectometer. The multiplexing of sensors is based on the time separation of signals. Requirements on the Bragg diffraction gratings that provide the linear dependence of the received signal on the FBG mechanical stress and temperature are determined. In the measurements of the FBG relative elongation and temperature, the threshold sensitivities are 0.8 × 10−4 (80 μstrain) and 5°C, respectively. Due to its simplicity and efficiency, the reflectometric method of FBG polling and multiplexing can be used to solve various measurement problems, in particular, the safety monitoring of the stressed-strained elements in building structures.

Intracavity and output spectra broadened in a Raman fiber laser with a low-Q cavity are measured and theoretically described. It is demonstrated that the previously developed approach of optical-wave turbulence can be used to describe the spectral broadening owing to the multiple four-wave mixing processes that involves numerous longitudinal modes of the low-Q cavity. The theoretically calculated spectra are in good agreement with the experimental data for various output reflection coefficients of the fiber Bragg gratings across the entire power range.

Stimulated Brillouin scattering (SBS) of single-frequency radiation in an AllWave™ telecommunication fibre and a phosphosilicate fibre is studied. The frequency shift and stimulated Raman gain are measured. The emission spectrum of a phosphosilicate Raman fibre laser is studied in the near-threshold regime. It is shown that SBS does not broaden the output emission spectrum of the Raman laser.

The possibilities of apodization of fibre Bragg gratings written by the holographic method with a Gaussian beam are considered. A simple method is demonstrated for smoothing the spectral profile of highly reflecting gratings by illuminating the written interference pattern by a homogeneous Gaussian beam.

The four-mirror cavity with a BBO crystal for frequency doubling in a wide-aperture argon laser is optimised. The dependences of the second-harmonic power on the displacement of a focusing mirror, the displacement of the crystal, and the discharge current are measured. These dependences are in good agreement with calculations. After optimisation, ~1 W of UV laser radiation at 244 nm was obtained with the conversion efficiency twice as large as that for the known similar lasers. It is shown that the increase in the efficiency was achieved mainly due to the increase in the discharge-tube aperture.

We have measured the shape of the Autler-Townes doublet and the peak of electromagnetically induced transparency (EIT) under plasma conditions. We compare the experimental results with the calculated spectrum of the probe field of a three-level ArII Λ -scheme by taking into account Coulomb collisions. We show that the Coulomb broadening of the EIT peak is small (less than 40%), while the saturation resonance is broad- ened under the experimental conditions by a factor of 3. In contrast to the saturation resonance attributable to the Bennett dip in the velocity distribution of the population, the EIT peak is a coherent effect and is broadened mainly through Coulomb dephasing. © 2004 MAIK "Nauka/Interperiodica".

The spectrum of a test field in a three-level ArII Λ scheme in the presence of a strong standing wave on the adjacent transition was measured. The known light-induced transparency peak was observed; the peak shifted as the detuning of the strong field was varied. In addition, a new resonance in the line center arose whose position was independent of the strong-field frequency. The resonance was caused by the higher spatial coherence harmonics on the test transition. Perturbation theory for a low and numerical calculations for a high standing wave intensity give qualitative agreement with experiment and substantiate the nature of the central transition.

Measurements of probe-field spectra in an N-configuration of rovibrational states of sodium dimers with two strong fields in opposite transitions are reported. From two to four peaks in the spectrum are observed, while theory has predicted up to six peaks. The discrepancy is explained by the absorption of the strong fields. A variation of the Rabi frequencies along the medium shifts the peaks and results in broadening of individual peaks and a reduction of the number of resolved components. A qualitative agreement with experiment is demonstrated. Additional mechanisms of broadening due to collisions and degeneracy of molecular states with large angular momentum are discussed.

An explicit solution is obtained for the four-wave frequency mixing ${\ensuremath{\omega}}_{d}={\ensuremath{\omega}}_{a}\ensuremath{-}{\ensuremath{\omega}}_{b}+{\ensuremath{\omega}}_{c}$ of two strong fields a and c and two weak fields b and d in a four-level system with large Doppler broadening in collinear geometry, where the frequencies of weak fields are nearly equal, ${\ensuremath{\omega}}_{b}\ensuremath{\simeq}{\ensuremath{\omega}}_{d},$ and the medium is optically thin. Without weak fields there are two independent two-level systems. A pair of weak fields probes two other allowed transitions. A peak of the mixing coefficient as a function of intensity is found around an equal Rabi splitting of both two-level systems. The effect is based on a resonance between two closed cycles of four-wave mixing via different dressed states. Three, four, or six peaks are predicted in the dependence of the mixing coefficient on the frequency of the weak field; two of them are a consequence of averaging over velocities. The model allows an interpretation of the dependence of the output wave power on the intensity and detuning in recent experiments on frequency mixing in sodium vapor.

An explicit solution is obtained for the four-wave mixing ω4=ω1−ω 2+ω3 of two strong fields E1, E3 and two weak fields E2, E4 in a four-level system with large Doppler broadening. Resonance of the intensity dependence of the mixing coefficient is found around equal Rabi frequencies, E1·d1=E3·d3, where d1,3 are the dipole moments of the corresponding transitions. The effect is interpreted as a crossing of quasi-energy levels. Up to 6 peaks appear in the dependence of the conversion coefficient on the detuning of the probe field E2. The unexpected additional pair of peaks is a consequence of averaging over velocities. The results permit interpretation of the saturation behavior found in recent experiments on mixing in sodium vapor.

Pump-to-Stokes and Stokes-to-Stokes relative intensity noise (RIN) transfer is discovered now for cascaded Raman fiber lasers (RFLs) with multimode laser pumping. We report on the first experimental study of this effect observed with two-stage phosphosilicate RFL and also propose its analytical explanation. It is shown that the peaks of radio-frequency (RF) spectrum associated with longitudinal mode beating in the pump laser cavity almost uniformly transfer to the first and second Stokes RF-spectra, thus increasing RIN of the RFL in megahertz-frequency domain.

A narrow interference resonance in the power of a single-mode ion laser as a function of the magnetic field is recorded and investigated. The nonlinear interference effect in the Zeeman laser is calculated taking account of Coulomb diffusion in velocity space. The two-photon resonance is found to differ substantially from the diffusion-free case.

It is known that dissipative solitons (DS) generated in fiber oscillators lose their stability at cavity lengthening. We report on the experimental realization of the DS oscillator with increased cavity length (L∼100 m). Stable operation is achieved in the all-fiber configuration consisting of a short single-mode-fiber (providing mode locking via nonlinear polarization evolution) and a long PM-fiber (providing formation of highly-chirped DSs) parts. We have identified the next limiting effect related to the onset of Raman conversion of the DS spectrum. The maximum output energy obtained with 5.5-µm-core PM-fiber amounts to ∼30 nJ in 30–70 ps pulses externally dechirped to 200–300 fs at repetition rate ≥1.7 MHz. At that, the Stokes pulse reaching comparable energy inside the cavity does not break the soliton stability, surprisingly. The obtained results are analyzed using a simple model. Higher DS energy is shown to be possible by means of a PM fiber core enlargement.

A fibre laser with a mirrorless cavity that operates via Rayleigh scattering amplified through the Raman effect is recently demonstrated. The properties of such random distributed feedback laser appear different from those of both traditional random lasers and conventional fibre lasers. In the present paper, basic mechanisms defining operation of such random distributed feedback (RDFB) fiber lasers are analyzed and the obtained experimental results are reviewed.

Raman gain spectral profile has been measured in a phosphosilicate fiber at high pump and Stokes (signal) wave powers. It has been shown that the profile saturates homogeneously. The main saturation mechanism is proved to be the pump depletion, i.e., Raman gain coefficient gR does not depend on the pump and signal wave power up to the level of ~ 3 W.

655 nm laser radiation with power of60 mW is generated by frequency doubling of a broadband randomly-polarized 1.31-microm Raman fiber laser (RFL). The red power appears to grow linearly with increasing RFL power up to 7 W at efficiency comparable with that for single-frequency lasers. It has been shown that multiple sum-frequency mixing processes involving different RFL modes provide the main contribution to the output, which is enhanced by 2 times due to the modes stochasticity.

For the first time it was experimentally demonstrated the interaction of core modes in 4-core fiber with large distance between the cores (28 mum). It was established that the effect of interaction is very sensitive to the fiber curving. The effect explanation based on indirect interaction of the cores inside the fiber through the central shell mode is given. The 70% energy transfer from one core to another was obtained for 40 cm long fiber.

We present results on characterization of lasers with ultra-long cavity lengths up to 84km, the longest cavity ever reported. We have analyzed the mode structure, shape and width of the generated spectra, intensity fluctuations depending on length and intra-cavity power. The RF spectra exhibit an ultra-dense cavity mode structure (mode spacing is 1.2kHz for 84km), in which the width of the mode beating is proportional to the intra-cavity power while the optical spectra broaden with power according to the square-root law acquiring a specific shape with exponential wings. A model based on wave turbulence formalism has been developed to describe the observed effects.

CW Raman fiber laser emitting at 954 nm under direct pumping by a high-power multimode laser diode at 915 nm is demonstrated. A cavity of the laser is formed with 2.5 km-long multimode graded-index fiber and two mirrors: highly reflective fiber Bragg grating (FBG) at one side and normally cleaved fiber end at the other side. The laser generates low-index transverse modes at the Stokes wavelength with output power above 4 W at a slope efficiency above 40%. It is shown that utilization of a narrowband FBG mirror with low reflectivity instead of the cleaved fiber end with Fresnel reflection leads to stronger spectral mode selection, but the generated power is reduced in this case.