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We present a novel electro-optically tuned external-cavity diode laser (ECDL) that utilizes a volume Bragg grating (VBG) as the frequency selective feedback element and a piece of high electro-optic coefficient Lead Lanthanum Zirconate Titanate (PLZT) transparent ceramic as the frequency tuning element. By adjusting the voltage applied onto the PLZT, a single-mode frequency tuning range of 2.5GHz without mode hoping is achieved. The laser wavelength is around 810.0nm with the line-width of 19MHz and the side mode suppression ratio (SMSR) of 37dB. The advantages of combining a VBG and PLZT transparent electro-optic ceramic is that the laser frequency can be tuned finely and rapidly in a range of longitudinal mode spacing without mode hoping. Moreover, the wide-range coarse frequency tuning of 32.7 GHz can also be realized by changing the angle of incident light beam.
©2011 Optical Society of America
1. Introduction
Tunable single-mode laser diodes are extensively used in high resolution spectroscopy, synthetic aperture laser radar and coherent optical communication systems. In these applications, single longitudinal mode operation, narrow laser linewidth, broad tuning range and rapid tuning speed are generally required. Using the optical feedback technique, such as scanning gas absorption peak, the tunable single-mode laser diode can also be applied to form a frequency stable laser source which is a fundamental part in laser cold atomic physics [1]. Littrow or Littman external cavity configuration with an angle variable diffraction grating [2,3] is one of usual ways to form this narrow width tunable laser, in which a very large tuning range (tens GHz ~several hundreds GHz) can be realized. Ref [3]. demonstrated a tunable Littrow external cavity laser diode with a mode-hop-free single mode tuning rage of 50 GHz at 793nm and tuning speeds of 1.5 GHz/μs. A kind of more compact ECDLs using volume Bragg grating (VBG) have been also reported. Though the use of the VBG is major in locking the wavelength of high power laser diode (LD) bars in earlier years [4,5], the application of VBG in narrowing linewidth of lower power LD has been demonstrated in recent years [6,7]. Compared with the diffractive grating ECDL, the VBG-ECDL structure has many advantages of small volume, symmetrical structure, polarization insensitive on efficiency, direction stability as the angle tuning. Currently, the tuning means of narrow linewidth VBG-ECDL is to utilize the piezoelectric transducer (PZT) mechanical adjusting. In Ref [6], an ECDL employing a micro-machined silicon flexure to sweep the laser frequency and a volume holographic reflection grating to provide the optical feedback was illuminated. The laser wavelength could be tuned from 780.2463 to 780.2379 nm (equivalent to 4.14 GHz) using PZT actuators integrated on the silicon flexure. Moreover, a frequency tuning range of 17.149 GHz could also be achieved by changing the VBG temperature. In Ref [7], the ECDL operates near 635 nm realized a PZT-controlled tuning range of 28 GHz and a 1-s linewidth of 900 kHz. The disadvantages of this PZT tuning means are its bad mechanical stability and low tuning speed. Electro-optic tuning method which is a common occurance in diffraction grating ECDL plays well in these aspects, but has not been used in the VBG-ECDL configuration until now, to our knowledge.
Transparent Lead Lanthanum Zirconate Titanate (PLZT) electro-optic ceramic has been demonstrated for the past decades and it represents a class of high performance electro-optic material with possess high electro-optic coefficient, good optical transparency, broad optical transmission window and low cost [8,9]. Therefore PLZT has been widely used in the optoelectronic device areas, including optical switch, optical modulator and attenuator. In this paper, we present a novel tunable ECDL which combines the superiorities of both the VBG-ECLD and the electro-optic tuning. By adjusting the voltage applied onto the PLZT ceramic, a single mode tuning range of 2.5G without mode hoping is achieved. The ECLD’s laser wavelength is around 810.0nm with the line-width of 19MHz and the SMSR of 37dB. The advantages for this proposed ECDL is that the laser frequency can be tuned finely and rapidly (less than 100) in a range of without mode hoping. Moreover, the wide-range coarse frequency tuning of 32.7 GHz to cover with multiple longitudinal mode spacings can also be achieved by changing the angle of incident light beam slightly. It is believed that this compact ECDL will have very good applications in the high coherent laser fields, such as atomic optics and coherent optical communication systems.
2. Design of tunable ECDL-VBG based on PLZT ceramic
The schematic diagram of the proposed electro-optically tunable VBG-ECDL is shown in Fig. 1 . The laser diode (Eagleyard Photonics, EYP-RWE-0840-06010-1500-ALN26-0000) fixed on a heat-sink is coated a high antireflection film. The measured center wavelength of the fluorescence spectra at room temperature (17°C) is around 810nm. The key components in the system are a PLZT electro-optic ceramic wafer (10mm × 2mm × 1mm) and a VBG (Ondax, 4mm × 4mm × 3mm). The PLZT ceramic has a high quadratic electro-optic coefficient of 1.1 × 10−16 m2/V 2 and a good optical transparency (larger than 95% with coating thin Al2O3 anti-reflection film in the ends). It will act as a modulation element of the cavity length by the electro-optic effect. The reflectivity and the 3-dB bandwidth of VBG at 810 nm is 50% and 0.08nm (~40GHz) respectively, which is considered as a frequency selective feedback mirror and the output cavity mirror. The light beam from the laser diode is collimated by an aspheric lens firstly. For the electro-optic efficiency of PLZT ceramic is relative with the polarization of the input beam, a λ/2 wave plate is used to adjust the beam polarization state to make it parallel to the electrical field direction. The DC stabilized voltage is applied onto the PLZT Ti/Pt/Au electrodes which are fabricated by a sputtered method. The transverse electro-optic effect is used in our system.
For the laser diode is coated an antireflection film, the mode of inter-cavity affects very small. Therefore the longitude mode space for the external-cavity mode can be expressed as:
where L(≈40mm) is the length of external cavity, L1(=10mm) and L2(≈30mm) are the length of PLZT and the rest length of the cavity respectively, n1(≈2.4) and n2(=1) are the index of PLZT and the air, c is the light speed in vacuum. The corresponding longitude mode space of ECDL is equal to 2.7GHz, which will decide the maximum mode-hop-free tuning range theoretically. In order to obtain the broader mode-hop-free tuning range, a shorter cavity length for the ECDL should be set up. Moreover, unlike the tunable diffractive grating ECDL whose spectral resolution is determined by the beam diameter, the spectral resolution of VBG-ECDL should be inversely proportional to the interaction length Lr [10]This beam diameter incense characteristic increases the possibility of much shorter cavity. Meanwhile, in our ECDL design, the beam diameter should be less than the PLZT’s aperture (about 1mm), which ensures the laser efficiency.
3. Experiment results and analysis
Based on the above design, we build the corresponding experimental setup as shown in Fig. 1. Due to the laser chip is coated the anti-reflection film, the output light is a wideband fluorescence spectrum around 810nm. Therefore, the central wavelength of the ECDL will be determined by the feedback peak of VBG. Figure 2(a) shows the laser spectrum for the proposed ECDL-VBG with an output power around 10mW. The central wavelength is 810.056nm and the SMSR is about 37dB, which is measured by an optical spectrum analyzer (ANDO 6037C) whose resolution is 0.01 nm. The laser linewidth is measured by the Fabry-Perot interferometer Series DL 100 with a free spectral range of 1 GHz and a linewidth resolution of 3MHz. The linewidth is around 19 MHz as shown in Fig. 2(b). Since the laser linewidth is much larger than the linewidth resolution of the Fabry-Perot interferometer, so this measurement is accurate. From Fig. 2(b), it is also easy to found that the laser is a good single-mode running.
Fig. 2 (a) Laser spectrum and (b) linewidth measurement by the Fabry-Perot interferometer with a free spectral range of 1 GHz and a linewidth resolution of 3MHz.
Changing the driven voltage applied onto the PLZT ceramic wafer, the mode shifting of ECDL-VBG will be happened which is also measured by the Fabry-Perot interferometer. The Fabry-Perot scanning method can only detect the relative frequency shifting but not the absolute frequency. When the DC driven voltage is increased from 544V to 860V, the mode-hop-free tuning of ECDL-VBG with a range of 2.5GHz will be obtained and this is shown in Fig. 3(a) . This tuning range is very close to the longitude mode space of ECDL-VBG (2.7GHz). Continue to increase the driven voltage, the mode-hoping phenomenon will be observed from the power shaking. In order to enlarge the mode-hop-free tuning range, one of the effective solutions will be to shorten the cavity length and make the whole cavity structure more compact. It should be also noticed that the PLZT ceramic wafer used in our experiment has a quadratic electro-optic coefficient, i.e. the electro-optic induced index change with the applied voltage satisfies [9]
where is electro-optic coefficient in the polarization direction along with the electric field, E is the applied electric field. Therefore, the mode-hop-free tuning of ECLD-VBG with the applied voltage will satisfy a good quadratic curve relation as shown in Fig. 3(b). The higher driven voltage has a larger frequency tuning radio. A theoretical simulation is also given and this is coincident with the experimental results basically.Fig. 3 (a) mode-hop-free tuning of ECLD-VBG and (b) laser frequency variation versus the applied voltage to PLZT.
A larger range of coarse tuning with mode hoping, which covers with multiple longitude mode spaces, may also be achieved by changing the incident angle of light beam slightly, such as using PLZT electro-optic deflector [11,12] or PZT mechanical adjusting VBG. In our experiment, a triangular electrode PLZT electro-optic deflector (as shown in Fig. 4(a) ) is used to replace the modulation element of cavity length in Fig. 1. Due the effect of electro-optic prism, the light beam will deflect in the interface with a very small angle [9, 11]. Different incident angle will induce the change of laser central wavelength. Figure 4(b) shows the shifting of the laser spectrum of ECDL-VBG from 810.030nm to 810.102nm. This is corresponding to 37.2GHz frequency tuning, which is limited mainly by the bandwidth of VBG (40GHz). In our following work, a combination of electro-optic phase and angle tuning in a PLZT ceramic wafer through the electrode design will be considered. By this means, the angle scanning obtains the coarse tuning of multiple longitude mode spacings and the phase modulation obtain the mode-hope-free tuning of a longitude mode spacing.
Fig. 4 (a) The electro-optic deflector based on PLZT ceramic for angle sweeping and (b) coarse frequency sweeping by changing the incident angle of light beam slightly.
4. Summary
We have proposed a novel electro-optic tunable VBG-ECDL by using high electro-optic coefficient PLZT transparent ceramic. Base on the quadratic electro-optic effect, a mode-hop-free range of 2.5GHz is obtained by changing the voltage applied onto the PLZT. This no mechanical moving parts configuration combined with series of VBG’s advantages makes this tunable VBG-ECLD a potential way form compact stable tunable laser source. Much larger coarse frequency tuning range of 37.2GHz can also be realized by designing the electrode form of the PLZT ceramic. We expect that this compact ECDL will have very good applications in the high coherent laser areas, such as atomic optics and coherent optical communication systems.
Acknowledgments
The authors want to acknowledge Prof. Aili Ding, and Dr. Xiyun He from Shanghai Institute of ceramics, CAS for providing the good performance PLZT ceramic sample. The work was supported by the key basic project of STCSM (Grant No. 09JC1414800), the National Natural Science Foundation of China (Grant No. 60807020 and Grant No. 61137010) and the Natural Science Foundation of STCSM (Grant No. 09ZR1435200).
References and links
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