A diode-pumped mode-locked double clad Yb3+-fiber ring laser with up to 118 nm bandwidth, a repetition rate of 7.6 MHz and a maximum output power of 2.8 W is presented. The broad bandwidth is achieved by suppressing the long wavelength components in the dispersive grating line of the resonator.
© 2002 Optical Society of America
Fast and accurate measurements with optical metrology systems like optical coherence tomography (OCT)  or optical coherence radar  require broad-bandwidth high-power optical sources. Superluminescent diodes, which are the common light sources for OCT systems, show about 40 nm bandwidth but only a few mW output power. For many non-medical applications this low output power limits the signal to noise ratio and the probe penetration depth on OCT measurements. As an alternative light source Bashansky et al. presented a Yb3+-doped superfluorescent fiber source with up to 700 mW output power with a bandwidth of 40 nm . The output power of this system is sufficient for many applications. But the resolution of OCT and coherence radar measurements are still limited to about 12.5 μm as the bandwidth determines the minimum achievable resolution of such metrology systems. Therefore there is a demand for sources with broader emission bandwidth and mode-locked laser systems are attractive candidates for such broad-bandwidth high-power light sources. The most prominent representative of those sources is the Ti:Sapphire laser, which is able to provide up to 400 nm bandwidth in combination with output powers of about 200 mW  or even higher output powers on the expense of smaller bandwidth. However, these laser systems are too bulky and too expensive for most medical and industrial applications, because they require argon-ion or frequency-doubled solid-state lasers as pump sources. Mode-locked Yb3+-fiber lasers do not suffer from those disadvantages, but typically the maximum bandwidth of these systems is below 40 nm with an output power of only a few mW , although Yb3+-fibers are able to provide gain over more than 60 nm [5,6]. As a consequence those fiber laser systems are up to now not used as light sources for metrology systems.
In this work we present a mode-locked diode-pumped Yb3+-fiber laser system which overcomes the above mentioned restrictions. The laser generated pulses with up to 118 nm bandwidth and showed a maximum average output power of 2.8 W. For this system the influence of the pump power and of the cavity dispersion on the pulse bandwidth was investigated. Broad-bandwidth operation was enabled by suppression of the long-wavelength components of the pulse spectrum inside the dispersive grating line of the cavity.
2. Experimental setup
Figure 1 shows the laser setup in detail, which consists of six modules: a 26 m long Yb3+-doped double-clad fiber that provides the gain and the nonlinearity, quarter- (QWP) and half-wave-plates (HWP) for polarization control, an optical isolator for unidirectional laser operation, a 50% output coupler (M3), a dispersive delay line with two parallel mounted diffraction gratings (1200 lines/mm) for compensation of the normal fiber dispersion and an adjustable blade inside the grating line. This blade was used for suppression of the long-wavelength components and was orientated parallel to the vertical grating lines. The double-clad fiber had a pump and laser core diameter of 4.2 μm and 400 μm, respectively, a Yb2O3-doping concentration of 6500 ppm and a cut-off wavelength of about 950 nm. The fiber was pumped at 975 nm by a 25 W fiber-coupled laser diode through the dichroic mirror M1 and was “kidney-shaped” coiled for improved pump-light absorption .
The output light of the fiber was steered by mirrors M1 and M2 to the optical isolator. The polarizing beam splitter at the input of the isolator rejected one polarization and, therefore, acted as an additional output port (port 2). Behind the isolator, the signal propagated below mirror M4 to the output coupler M3 and then to the dispersive grating pair unit. The total dispersion of this unit was adjusted by the grating separation. After passing the second grating, the different wavelengths of the signal propagated spatially parallel and were separated laterally. Therefore, the horizontally adjustable (vertical) blade behind the second grating acted as a spectral filter. The portion of the spectral pulse components which passed this blade was then back reflected below the incoming beam by a 90° prism and then coupled into the Yb3+-fiber via mirrors M4 and M5. The polarization at the fiber input was determined by the settings of the zeroth-order wave-plates (1.064 μm). For appropriate wave-plate settings the polarization beam splitter (port 2) in combination with the nonlinear polarization rotation in the fiber rejected low intensity signals like a saturable absorber and self-starting mode-locking with a repetition rate of 7.6 MHz was achieved .
3. Results and discussion
The pump power for self-starting mode-locking of our fiber laser depended on the wave-plate settings, on the total cavity dispersion and especially on the blade position inside the dispersive grating line. Without spectral filtering the mode-locking started typically at about 4.5 W pump power. The output signal showed a center wavelength of 1.1 μm and a maximum bandwidth of about 40 nm. Moving the blade into the beam inside the grating line suppressed the long-wavelength components and resulted in a shift of the center wavelength of the circulating pulse to shorter values. If the blade was moved further into the beam the bandwidth of the output signal increased dramatically for appropriate wave-plate settings. At an optimum position an extremely broadband output signal with up to 118 nm bandwidth (FWHM) extending from 1060 to nearly 1240 nm was achieved. In order to determine the cut-off wavelength at that specific blade position, we measured the ASE-signal of the Yb3+-fiber behind the grating line with and without the blade. As a result, only wavelengths below 1080 nm (13 dB) are transmitted, which is a surprisingly small part of the broad bandwidth signal.
Moving the blade further into the beam reduced strongly the signal bandwidth as well as the center wavelength. It should be noted that moving the blade from the other direction into the beam in order to suppress the short-wavelength components shifted only the pulses to longer wavelength and no spectral broadening was observed.
In Figure 2a the normalized output spectra of the two ports of the laser system are shown, adjusted for maximum bandwidth at port 2. At a pump power of 18.8 W we achieved output powers of 0.55 W at port 1 and 1.31 W at port 2, respectively. The bandwidth (FWHM) of the output spectra at port 1 was 103 nm and 118 nm at port 2, which is about 3 times larger than for a Yb3+-fiber laser system without spectral filtering. Both curves are similar in shape, except around 1175 nm, where the spectral intensity measured at port 1 was negligible, whereas the output of port 2 showed a local maximum. This was probably caused by the limited bandwidth of the zeroth order (1064 nm) QWP in front of the isolator, since the signal polarization at the input of the isolator determines the power splitting ratio between port 1 and port 2.
The filtering blade acted as a short-pass filter. The measured spectral attenuation of this filter was about 40 dB at 1087 nm and increased further with wavelength. Therefore the long-wavelength components of the output spectra could not circulate inside the cavity and must be generated in a single pass through the Yb3+-fiber by various nonlinear effects like self-phase modulation, four-wave mixing and stimulated Raman scattering .
At shorter wavelengths a steep decrease around 1060 nm was observed. This behavior was nearly independent of the wave-plate settings, the pump power and the output port. This limited emission of short-wavelength components was caused by the reflection characteristics of the dichroic mirror M1, which was highly transmittive for wavelengths below 1055 nm and additionally, reabsorption in the Yb3+-fiber is increasing with shorter wavelengths, what may act as an additional long pass filter.
The influence of the pump power on the measured bandwidth at the two output ports is shown in Figure 2b. For pump powers of up to 15 W the bandwidths increased nearly linearly with similar values for both ports. This can be easily be understood in terms of the intensity dependent nonlinear effects, which are broadening the spectral bandwidth. In the pump power range from 15 to 19 W the maximum signal bandwidth was achieved. At these power levels the maximum bandwidth of the spectrum at port 2 was at least more than 10 nm broader than at port 1. For pump powers beyond 19 W the bandwidth of both output signals decreased to values below 100 nm.
A more detailed investigation, by using a fast photodiode (2 GHz) and an oscilloscope (500 MHz), showed typically only one short pulse with a repetition rate of 7.6 MHz for pump powers below 20 W. For higher pump powers typically two closely spaced short pulses were observed. The decreasing bandwidth with increasing pump power at this power level was probably caused by multiple pulse operation which reduced the peak intensity inside the fiber.
In Fig. 3 the measured autocorrelation of a 116 nm bandwidth signal from port 2 is shown. It is noticeable that the autocorrelation function shows not the typical 8:1 ratio near the interference pattern. Even for a time delay of 30 ps the autocorrelation signal intensity was still about 25% of the interference maximum. This large pedestal is probably caused by a large chirp of the output signal. Taking the large bandwidth of the signal and the fiber dispersion into account a pulse duration of up to 100 ps could therefore be expected at the output.
It should be noted, that in our experiments we observed no influence of the total cavity dispersion on the spectral bandwidth between -1.4 × 105 fs2 and 1.6 × 105 fs2. Measurements of the output spectra at four different values of the second order dispersion between this values showed always the extreme spectral broadening with similar signal bandwidths.
In Fig. 4 the output power of both ports are plotted versus the pump power for two different adjustments of the wave-plates and the filtering blade. In Fig. 4a these results are shown for an optimized spectral bandwidth, spanning up to 118 nm at port 2. For this bandwidth an output power of 1.3 W at port 2 was achieved. A further increase of the pump power enabled a maximum output power for this set-up of 1.02 W at port 1 and 2.16 W at port 2 but at reduced signal bandwidth (see Fig. 2b). At the bandwidth maximum of 118 nm the output power was 1.3 W at port 2. In Fig. 4b the laser system was optimized in respect of the output power, which have maximum values of 0.8 W and 2.8 W, respectively, with a bandwidth of 85 nm. In both figures the gradients of the curves were smaller at lower pump power levels. This power characteristics were mainly caused by a shift of the pump wavelength with pump power. This resulted in a variation of the pump light absorption since the absorption peak of Yb3+ at 975 nm is relatively narrow .
This unusual and hardly expected result of extreme spectral broadening of the Yb3+-fiber laser signal by nearly a factor of 3 compared to previous reported systems can be interpreted in the following way:
The long wavelength spectral components at the output above about 1090 nm must be generated inside the fiber each pass, since these components were blocked by the spectral filter before the fiber input. Therefore these spectral components were obviously generated inside the fiber by nonlinear effects like self-phase modulation and four wave-mixing and probably in a smaller extent by intra-pulse stimulated Raman scattering. In addition to the generation of new spectral components by nonlinear effects the generated long-wavelength components were partially further amplified by the fiber gain. On the other hand short wavelengths have a lower gain or were even absorbed by the Yb3+-fiber. Nevertheless, this laser system showed not such a extreme spectral broadening without spectral filtering.
The spectral filtering reduced the pulse duration at the fiber input, reduced the influence of the initial chirp, the gratings and the highly uncompensated third order dispersion on the input signal and increased the gain in the fiber. The combination of these effects raises the peak intensity inside the fiber. Because of the higher peak intensity the above mentioned intensity dependent nonlinear effects generate the observed additional spectral components in combination with the spectral filtering.
The spectral filtering reduces the pulse duration at the fiber input since the signal is there strongly chirped. As this is in contrast to the common known pulse lengthening with bandwidth reduction, we want to explain it in more detail. The large chirp is attributed to the passive mode locking based on the nonlinear Kerr-effect in the fiber and the large fiber and grating dispersion. A result of the passive mode-locking mechanism is that the circulating pulse has the lowest chirp roughly in the middle of the fiber and is strongly chirped at the input and the output of the fiber. For example the estimated pulse duration at the fiber input is roughly about 400 times above the bandwidth limit for a 40 nm bandwidth signal, taking the measured fiber dispersion of 2.35 × 104fs2/m into account. Therefore the different spectral components of the strongly chirped pulse are spread over the large pulse length and the time-bandwidth product is far above the theoretical limit. Under these circumstances a reduction of the spectral bandwidth by the above mentioned spectral filtering reduces the pulse duration at the fiber input, since this separates excessive spectral components and therefore one of the pulse wings. This pulse width reduction by spectral filtering can be driven until the time-bandwidth product is approaching the theoretical Fourier limit.
If the filtering is further increased the pulse approaches the Fourier limit and the pulse duration increases with decreasing bandwidth. At this regime the remaining pulse is nearly time-bandwidth limited. Because of the relatively small bandwidth the influence of the initial chirp, the gratings and the high uncompensated third-order dispersion in the cavity of about 2.7 × 106 fs3 was also substantially reduced. Hence the effects of the initial chirp were blurred and the pulse duration depends mainly on the remaining bandwidth. Probably this also is the reason why the grating dispersion showed no influence on the spectral broadening. It should be noted that without spectral filtering a bandwidth limited pulse is not expected somewhere inside the fiber laser because of the highly uncompensated third order dispersion.
Furthermore the spectral filtering increased the cavity loss, in particular for the long wavelength components and reduced therefore the power at the fiber input. As in laser operation the gain equals the losses, the spectral filtering resulted in a higher gain. Furthermore the center wavelength of the input signal is shifted to the short-wavelength side of the gain maximum. Therefore the gain for long-wavelength components of the pulse is additionally increased. However our experiments with high cavity loss without spectral filtering showed, that a high gain is not sufficient for an extreme spectral broadening.
The above described relations and effects explain qualitatively the observed behavior. Nevertheless, the detailed interaction between the various nonlinear effects, the Yb3+-gain and the fiber parameters is quite complicated and has to be further investigated by numerical simulations and experiments.
To our knowledge, we have demonstrated the largest bandwidth (118 nm) of a mode-locked Yb3+-fiber laser. This high power laser source (up to 2.8 W) is well suited for optical measurement techniques, like optical coherence tomography or optical coherence radar, because of the broad bandwidth signal generated in a single mode fiber (950 nm cut-off). By using Yb3+-fibers with larger mode field diameters further power scaling of this broad bandwidth system should be possible.
This research is supported by the German Ministry of Science, Education, Research and Technology under contract 13N7799. We gratefully acknowledge the Institut fuer physika-lische Hochtechnologie Jena (IPHT) for preparing the Yb 3+-double-clad fiber.
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