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We propose and demonstrate a tunable Q-switched erbium doped fiber laser with a digitally controlled micro-mirror array device. The tunable and pulsed output of the laser was achieved by the pixelated spatial modulation of the micro-mirror array. The wavelength tuning from 1530 nm to 1555 nm was shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate.
©2006 Optical Society of America
1. Introduction
The optical fiber lasers have been studied extensively as an essential optical source for wavelength-division-multiplexed transmission systems and for performance testing of optical components. The broad gain spectrum of erbium doped fibers (EDFs) has generated considerable interest and has resulted in extensive development of single-frequency tunable fiber lasers. To date, several wavelength tunable erbium doped fiber lasers (EDFLs) have been demonstrated with wide tuning ranges using different wavelength-discrimination devices [1–8]. In addition to the wavelength tunability, the pulsed laser operation of EDFL has attracted increasing interest in nonlinear optical applications [9]. Recently, micro-electro-mechanical system (MEMS) technology has drawn considerable attention due to its potentials of optical switching, interconnecting and spectral filtering processing [10–15]. Also, this MEMS technology provides possibilities for variety of novel applications in laser systems and the digital micromirror device (DMD) based tunable fiber laser design was firstly proposed [16]. The regularly arrayed micro-mirrors have a spectral filtering characteristic by reflecting the spectrally diffracted light with selected micro mirrors. Also fast switching response of the micro-mirror array can be applied to Q-switch operation of lasers.
In this paper, we report a tunable and pulsed EDFL exploiting the electronically controlled digital micro-mirror array (DMMA) for Q-switching and tuning a lasing wavelength. The two distinct architectures were proposed for the flexible wavelength tuning and pulsed laser operations, and the characteristics of proposed EDFLs were experimentally demonstrated.
2. Tunable Q-switched EDFL based on a single micro-mirror and a tunable filter
Fig. 1. Schematic of the tunable Q-switched fiber laser with a single micro-mirror and a tunable filter.
The configuration of the proposed tunable Q-switched EDFL based on the DMMA is shown in Fig. 1. The laser cavity was formed in the ring structure and an erbium doped fiber amplifier (EDFA) was employed as a gain medium. The EDFA has a saturated output power of 13 dBm and more than 30 dB small signal gain over the signal wavelength range from 1530 to 1562 nm. The tunable interference type filter was used for tuning the lasing wavelength from 1540 to 1560 nm. Its transmission bandwidth and insertion loss are 0.8 nm and 3 dB, respectively. The fiber polarization controller and the output coupler were utilized for optimization of laser output and the optical isolator was placed for maintaining unidirectional ring laser cavity. The DMMA (DLP™ in Texas Instrument, the DMD part number : *1076-46c) was used as a switching element for cavity loss modulation through the reliable mirror rotation. The DMMA consists of individually addressable aluminum micro-mirrors whose mirror size is 13.68 µm. The array size is 1024×768 in square grid pixel arrangement and the glass window of DMMA is optimized for near infrared wavelength from 900 nm to 2000 nm. Due to the high coupling loss between objective lens and the DMMA, the total loss of laser cavity was about 4 dB. In Fig. 1, the incident light from the port 2 of circulator returns back to the laser cavity in the flat state of mirrors (ON state) and at the tilt state of mirrors with angle of +12 degree to the normal direction of mirrors (OFF state), the incident light deflects away with a specific angle of direction. Therefore cavity loss of the laser could be modulated by switching the state of micro-mirrors between ON and OFF states. In this scheme, in order to avoid the coupling loss due to the diffractions from regularly arrayed micro-mirrors, the light from port 2 of circulator was focused on a single micro-mirror in the DMMA by using an objective lens following the collimator.
Fig. 2. (a) Switching response of the DMMA at the repetition rate of 66 Hz. (b) The Q-switched pulse train of the laser when the pump current was 70 mA.
Fig. 3. (a) Typical Q-switched pulse waveform for modulation frequency at 66 Hz. (b) Corresponding optical spectrum.
The spectral shape and the pulse oscillation behavior of the laser were measured by detecting the lasing signals at both ends of 3 dB coupler with an optical spectrum analyzer and an oscilloscope following the high speed photodiode whose peak response is 0.95 A/W at 1550 nm wavelength. The basic principle of Q-switching is similar to that described elsewhere [17, 18] and relies on the intensity modulation in the laser cavity. To initiate a pulsed laser operation, the cavity loss was modulated by the micro-mirror which was actuated with a period of 15 msec (due to the control speed limit of processor in the flat state of the micro-mirror) and a backward reflection (ON state) time of 400 µsec. The measured modulating response of micro-mirror is shown in Fig. 2(a). When the light was reflected by ON state of the micro-mirror, the fiber-cavity Q switches from low to high value.
In such a configuration, the wavelength of laser could be swept by the tunable filter within the amplifier gain bandwidth, and we were able to get pulses of which the peak powers are about thousands times higher than the average power of the continuous emission. In Fig. 2(b) and Fig. 3(a), Q-switched pulse train and pulse waveform of the laser are shown when the pump current was at 70 mA. As shown in Fig. 2(b), a stable repetition rate of pulse train was observed. The repetition rate of pulse train was measured about 66 Hz and the pulse width at half maximum was about 3.21 µsec. Corresponding optical spectrum of the laser in Q-switch mode is shown in Fig. 3(b). The laser emitted a relatively narrow linewidth less than 0.23 nm at 70 mW pump current and the peak wavelength was about 1549.6 nm which could be controlled. As shown in Fig. 4, the average output power grew linearly with increasing pump power. The average output power of ~107 µW was limited by the damage threshold of a micro-mirror for the tightly focused light. The threshold of Q-switching operation of the laser was 55 mA of pump current near 1549.6 nm wavelength.
3. Electronically tunable and Q-switched EDFL with a DMMA and a diffraction grating
By utilizing the spectral dispersion of diffraction grating and the pixelated spatial light modulation property of DMMA, both wavelength tunable and pulsed operations of the EDFL could be realized simultaneously. The schematic of the proposed electronically wavelength-tunable and Q-switched fiber laser is shown in Fig. 5. The laser cavity was formed in the ring structure with an EDFA as the same configuration of the tunable Q-switched EDFL with a single micro-mirror in the previous section. In this configuration, the DMMA acts as both a wavelength tunable filter for selecting lasing wavelength and a switching element for cavity loss modulation. In Fig. 5, the collimated light from the port 2 of circulator is launched to diffraction grating and it is spectrally spread out by the diffraction grating (600 grooves/mm, ruled; Optometrics LLC) in the first order. The aspheric lens included collimation package was used as a collimator in this experiment. The output beam diameter of collimation light is 2.07 mm at 1/e2 and full-angle beam divergence is 0.055°. The distance between collimator and diffractive grating was about 50 mm. The diffracted rays from the diffraction grating are re-collimated by the following biconvex lens with a focal length f=51 mm, and then horizontally projected on the DMMA depending on their own wavelengths. The beam at the surface of the DMMA has a line shape with ~19 mm length. Due to the unattainable pixelated state control of mirrors in flat state, the DMMA was inclined with 12 degree against the vertical direction for maximizing the reflection efficiency of spatially modulated light with a bistable ±12° tilt angle of mirrors. When mirrors are tilted with -12 degree (in the negative tilt state), the incident light on the micro-mirror array is deflected to the outside of cavity, and in the tilted state of mirrors with +12 degree (in the positive tilt state), the light is reflected into the laser cavity for lasing. Therefore, this mosaic of discrete switching elements can be constructed as a wavelength selecting spatial light modulator. From this pixelated spatial light modulation, not only lasing wavelength control but also cavity loss modulation for Q-switching operation could be achieved by selecting positions of reflecting mirrors and fast tilting motion of the micro-mirror array.
The low insertion losses are obtained with the DMMA by matching diffractive effects with mirror tilt angle giving rise to a switched blazed grating (SBG) device. The total integrated reflectivity of a mirror array is a function of the area of the mirrors constituting the array, the angle of incidence and reflectivity of the mirror material at a specific wavelength. The DMMA behaves like a diffraction grating with the maximum power reflected or diffracted in a direction of θr, relative to the surface normal, determined by the pixel period(d), the wavelength (λ), and the angle of incidence θi. The maxima in the reflectivity distribution function is governed by diffraction equation, d(sinθr+sinθi)=nλ, Where n is the order of diffraction. The condition in which the direction of incidence and diffraction are identical is referred as the Littrow configuration, and the diffraction equation reduces to the well-known Bragg equation, 2dsinθ=nλ. The tilt angle of the mirrors strongly controls the reflective power. The Fraunhofer diffraction in the Littrow case directs the light into a ray with an angle equal to the angle of incidence (θi=θr). When the angle of the Fraunhofer diffraction is equal to a diffractive order, the DMMA is said to be blazed, and most of the diffractive energy can be coupled into a single diffraction order. Under these conditions, the optimized pixel period(d) and tilt angle of mirror are estimated 13.8 µm and ~9° tilt angle in conventional band (C-band), respectively. However, the micro-mirror array using in this experiment is designed with d=13.68 µm square micro-mirrors operating in a bistable ±12° tilt angle, thus it is difficult to create a high diffraction efficiency blazed grating condition for infrared C-band operation. Total loss of the proposed laser cavity was measured about 17 dB including the diffraction efficiency and reflective loss of micro-mirror array at 1545 nm wavelength. The total laser cavity loss was decomposed into 3 dB loss of grating, 6 dB loss of DMMA, ~6 dB loss of free space coupling, and 2 dB loss of ring pass in the laser cavity. The high insertion loss of the proposed laser cavity can be reduced with the optimization of free-space optic designs and matching diffractive effects with well-fitted tilt angle of mirrors.
Fig. 6. Mosaic of mirror pixels in the DMMA. The DMMA consist of 1024 x 768 individually addressable mirror pixels. (Blue shaded area represents the positive tilt state of mirror columns for selecting the lasing wavelength)
Fig. 7. (a) Spectral reflectivity of the diffraction grating-DMMA. (b) Response function of the mirror column (18×768) at 550 pixel address.
In order to select the lasing wavelength and to control amount of light directed to the laser cavity, the positively tilted area was vertically formed in a thin rectangular shaped pattern with a 18×768 section of mirror mosaic for the selective reflection of diffracted light. Figure 6 shows mosaic of mirror pixels in the DMMA used for the tunable Q-switched EDFL. The blue shaded columns are positive tilt area for selecting the lasing wavelength as mentioned above. The number of mirrors determines intensity and spectral resolution of the reflected light. As the width of columns increases, the reflection power increases but the spectral resolution of the laser decreases. Therefore the appropriate width of mirror columns should be determined for the compromise between the power of lasing signal and the spectral tunability. It was found that 18 columns of mirrors were optimized width for the laser having fine spectral tunability. It corresponds to about 1.5 nm in the spectral dispersion plane of the DMMA. When all mirrors of the DMMA are in positively tilted state, the reflective characteristic of the DMMA was measured with amplified spontaneous emission (ASE) of EDFA and shown in Fig. 7(a). Due to incomplete collimation of light and aberrations of a lens, the reflection of the DMMA has spatially different reflectivity according to the wavelength. In Fig. 7(a), almost incident light is reflected near 1545 nm wavelength and the reflectivity of the DMMA is rapidly reduced at the both side wavelengths at 1530 and 1557 nm. The spectral response of positively tilted 18 columns of DMMA was measured at 550 pixel-address and is shown in Fig. 7(b). The transmission peak of column mirror is located near 1540 nm wavelength and there is typically about 25 dB extinction ratio between positive tilt state versus negative tilt state. With a fast setting speed about ~15 µsec, the DMMA is well suited for dynamically tuning the wavelength of the laser system.
Fig. 8. (a)The laser output spectra of proposed laser along the center position of reflecting mirror columns from 150 to 800 pixel-address. (b) The tuning lasing wavelength versus the reflecting mirror array position.
The tunable function was achieved by sweeping the pixel positions of positively tilted mirror columns in the DMMA. When the center position of reflecting mirror columns moved from 150 to 800 pixel-address (one pixel equals to one mirror) in 50 pixel interval, the laser output spectra of the proposed laser were measured and is shown in Fig. 8. As the center position of reflecting mirror columns is changed, the lasing wavelength is linearly varied from 1530 nm to 1555 nm. It can be seen that the signal-to-noise ratio is better than 50 dB within the whole tuning range of 25 nm, and the linewidth of each spectral peak is about 0.17 nm with 0.01 nm resolution of the OSA. The tuning resolution of lasing wavelength versus the reflecting mirror array position was calculated about 0.037 nm/pixel and the measured minimum wavelength selectivity was about 0.1 nm with 3 pixel-address change of the center position of reflecting mirror columns. The output power of laser was varied with its lasing wavelength due to the overall gain spectrum and the maximum power difference between lasing peaks was about 10 dB. The Q-switched laser operation of the proposed tunable laser was also achieved with fast switching of the mirror state of selected columns. For the pulsed laser operation, the states of mirror columns, which have initially the positive tilt state for selecting the lasing wavelength, were switched to the negative tilt state with a period of 7.74 msec. The modulation response of mirror columns in Fig. 9(a) lead to Q-switching operation. The typical pulse waveform and the pulse train of laser are shown in Fig. 9(b) when the pump current of EDFA was 200 mA and the center position of reflecting mirror columns was located at 450 pixel address. The pulse train showed a stable repetition rate and the measured actual repetition pulse rate of the laser was about 130 Hz. The pulse width at half maximum was about 40 µsec and the corresponding optical spectrum of the laser in the Q-switching mode is shown in Fig. 9(c). A relatively narrow linewidth less then 0.7 nm was observed and the center wavelength of the laser was about 1543.9 nm.
Fig. 9. (a) The modulation response of the DMMA with a period of 7.74 msec. (b) The typical output pulse train and pulse waveform of the tunable Q-switched fiber laser. The center position of reflecting mirror columns is 450 pixel-address. (c) Corresponding optical spectrum.
4. Summary
We have proposed what we believe is the first demonstration the wavelength tunable Q-switched EDFL using a digitally controlled DMMA. Two laser architectures were constructed and their performances were evaluated. For the realization of a proposed fiber laser, DMMA was used as a switching, or a switching and wavelength selective device for the pulsed wavelength tunable laser operation. As a preceding experiment for the DMMA based electronically tunable and Q-switched fiber laser, the feasibility of DMMA as a fast switching element was confirmed in Section 2. By deploying a single micro-mirror and the tunable filter, the tunable Q-switched EDFL was demonstrated with a short pulse width about 3.2 µsec and stable pulse train, emitting a relatively narrow laser line width of less than 0.23 nm. The center wavelength of the laser could be also controlled in C-band. In Section 3, by fully utilizing spatial modulation property of the DMMA, the electronically tunable and Q-switched EDFL was presented. The lasing wavelength could be flexibly tuned by properly adjusting the pixel-address of reflective section in the DMMA. Also the pulsed laser operation was implemented with modulation of mirror states of the DMMA. The flexible wavelength tuning from 1530nm to 1555 nm has been shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate. By further optimizations of the blaze condition of the DMMA and the structure of a laser cavity, we believe that these tunable Q-switched laser schemes could be used for developing the highly desired tunable fiber laser systems for the variety applications in optical communication and instrument systems.
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