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The cascaded asymmetric exposure long-period fiber gratings are fabricated by CO2 laser, which provide multi-wavelength filters with anisotropic transmission spectrum under different states of polarization. Inserting this device in the ring cavity of an erbium-doped fiber laser, a triple-wavelength switchable lasing laser with equal spacing of 2.6nm has been obtained. Owing to the polarization dependent loss of the new cascaded long-period fiber grating, the wavelength switching of random combination of C 1 3, C 2 3 and C 3 3 is demonstrated through the polarization controlling. We derive the wavelength switch to the polarization characteristic of cascaded asymmetric exposure long-period fiber gratings.
©2007 Optical Society of America
1. Introduction
Multiwavelength switchable fiber laser attracts many interests in recent years due to its narrow line width, high output power, high stability and easy compatibility with the fiber system. Not only the multiwavelength fiber lasers are suitable for the wavelength division multiplexing (WDM) system, but also they can be used in the spectroscopy, two-photon microscopy [1], fiber optic gyroscope, fiber sensor system and optic devices testing, especially the switchable fiber laser is an excellent candidate light source that adapts the WDM system wavelength routine.
One of the problems with the common multiwavelength switchable erbium-doped fiber laser (MS-EDFL) is the homogeneous broadening characteristic of the EDF at room temperature, which generates strong mode competition. This property limits the number of the oscillated wavelength and switch combination possibility. Various methods are used to overcome this drawback. Up to date, the ways to achieve the wavelength switching mainly includes: semiconductor optical amplifier (SOA) [2], polarization hole burning (PHB) such as polarization maintaining fiber (PMF) based Sagnac loop cavity [3] and polarization dependent loss element (single long-period fiber grating on highly birefringent fiber) [4], cascaded fiber Bragg grating [5] or sampled fiber Bragg grating, fiber Bragg grating written in PMF [6] or few-mode fiber [7] and acoustic waves [8].
Long-period fiber gratings (LPFGs) are band-rejected optical fiber devices fabricated by periodic refraction index change along the fiber, which transfer light from the fiber core mode to the cladding modes. Typically, the pitch of the grating is several hundred microns. Carefully investigation of the grating fabrication method shows that the polarization characteristic of the grating can not be neglected, especially using the CO2 laser and femtosecond laser in fabrication [9,10,11], the origin is generally considered to be caused by single side-exposure method which exposes only one side of the fiber to the laser during the fabrication process. The polarization-dependent loss (PDL) is always defined as a symbol of polarization. Detailed research into the polarization of the LPFG proves asymmetric mode coupling such as coupling between LP01 and LP1n or the hybrid cladding modes. Once it was considered as a major disadvantage for the application of the LPFGs, and several techniques have been proposed to decrease the PDL [12,13].
In this paper, we report a new method of using the CO2 laser fabricated cascaded asymmetric exposure long-period fiber grating (CA-LPFG) on a normal single mode fiber (Corning SMF28) in the cavity of MS-EDFL. The spectrum change of CA-LPFG under different state of polarization (SOP) is demonstrated, which is considered to switch the laser wavelengths. Based on the PDL of the new CA-LPFG, a triple-wavelength switchable fiber laser with a simple ring configuration has been demonstrated.
2. CA-LPFG and its polarization characteristic
Cascaded long-period fiber gratings (CLPFGs) provide multiwavelength filtering because of the interference between core modes and cladding modes that referred to as in-fiber Mach-Zehnder phenomena [14], the PDL increase of the CLPFGs can be explained by the following calculation.
For a single LPFG, the PDL induced by asymmetric laser irradiation can not be neglected, consider the grating as a polarization element, then the propagation though the grating can be described as a matrix T
In the above equation, the x-y coordinate is set to the principle loss axis and no reflection is considered as for LPFGs. a and b, ϕ1 and ϕ2 are transmission and phase factor for x and y directions at a certain wavelength λ, respectively.
For an input electric field Ein = [Ex Ey exp(j(φ)]T, where Ex and Ey are the electric field amplitude of the x and y directions, respectively, and φ is their phase difference.
For two cascaded LPFGs, the total transmission matrix is described as below:
For comparison, at a certain wavelength λ, the input field though a single LPFG, the PDL (refer to PDL of a single LPFG) is (assuming a>b)
And though the CLPFGs,
Where it shows the PDL of the CLPFGs is equal to 2 times of the PDL of a single LPFG, which means in the polarization use, the CLPFG is more efficient. It is easily to extent the equation to the number of LPFGs for cascading number to n, the PDL is PDLn = 2n * PDL 1. We call it the “cascaded enhancement of PDL”. It must be emphasized here that the above analysis is based on the same single LPFGs cascaded at the same polarization directions. In our experiment, it is realized by exposing the same fiber side (which in the Fig. 1 referred to the phototropism side) to the laser induce the same asymmetric index change, no torsion or twisting.
Fig. 1. Asymmetric exposure during the fabrication of cascaded long-period fiber gratings. The phototropism side shows larger index change than the negative phototropism side, and it induces new polarization properties to the device.
The CA-LPFG used in the fiber laser is fabricated by spot focused continuous CO2 laser with output power of 0.6W, and the diameter of the focused laser spot is 140μm, time duration of the exposure is 0.5 second. The Corning SMF28 single-mode fiber is put on an electric-controlled moving stage with a heavy weight of 15g. The pitch of the grating is 625μm, and the separation between two same gratings is 28cm, which results in the interference spectrum with a fringe spacing of 2.6nm.
We set up a spectrum test for CA-LPFG under different polarization conditions as in Fig. 2(a). The schematic contains an amplified spontaneous emission (ASE) light source from 1510nm to 1600nm, a fiber polarizer which change the light to a certain SOP, a polarization controller (PC) and the optical spectrum analyzer (OSA).
By adjusting the PC, the output spectrum with different SOP is observed by the OSA. The results are showed in Fig. 2(b); three different colors represent three different SOP while the green one is the spectrum of a single LPFG. Maximum transmission power change within the range of 1550nm and 1565nm (magnified insert window) is 2.75dB, and maximum dip movement is 0.38nm. Compared to SNR of about 10dB and equal peak-to-peak spacing of 2.6nm, they should be taken into consideration especially the transmission power change. And more important, we can easily obtain from the figure that the spectrum change with SOP is anisotropic for different wavelength, which will lead to the different fiber laser output.
Fig. 2. (a)Experiment setup for testing the spectrum change under different SOP. The fiber polarizer turns the light from the ASE source to linear polarized, the PC is using for controlling the SOP in the fiber. (b)Transmission spectrum change with 3 different SOP and the spectrum of a single LPFG, the insert figure is the detail situation around 1550nm-1565nm.
Here we give a reason that the polarization-dependent spectrum change is not so attractive. Firstly, the fiber we used is the normal Corning SMF28, the special polarization characteristic induced by asymmetric exposure method and cascaded enhancement is not so huge, also seen in the previous paper [13]; secondly, the polarization test is not perfect because the linear polarization light generated by the fiber polarizer will quickly change to the elliptical polarization state or even worse after propagating though a certain distance fiber, which clearly affects the results.
3. The switchable triple-wavelength laser output
The MS-EDFL is set up as in Fig. 3, in which the 16m EDF with a doped concentration of 400 ppm is pumped by a 980nm source with a power of 100mW, after a polarization-dependent isolator which polarized clockwise light and a PC controlled CA-LPFG, the 10% output laser is monitored by an OSA, and the other 90% is cycled into the loop laser.
By adjusting the PC, different status of output laser is realized. As illustrated in Fig. 4, single-wavelength laser and dual-wavelength laser with a combination of the three equal spacing wavelength 1556.0nm, 1558.6nm and 1561.2nm (C 1 3 and C 2 3) are observed. The single-wavelength laser has a peak power of −20.7dBm, both in the Fig. 4(a), Fig. 4(c) and Fig. 4(e); while the two peak power in the Fig. 4(b), Fig. 4(d) and Fig. 4(f) are −21.4dBm and −20.9dBm, respectively. The signals to noise ratio (SNR) of the single- and dual-wavelength lasers are larger than 40dB. And the triple-wavelength output of laser (with the combination of C 3 3) is showed in Fig. 4(g), the maximum peak power difference is 3.5dB, and average SNR also maintains as large as ∼40dB.
Fig. 4. Switch of the laser output for one wavelength ((a), (c), (e)), dual-wavelength ((b), (d), (f)) and triple-wavelength with corresponding transmission spectrum ((g)).
The experiment results are analyzed as follows:
Here, the erbium-doped fiber laser with 16m EDF has a maximum net gain at the wavelength around 1560nm, the laser oscillation occurred around 1560nm. The mode competition plays an important role in the laser performance, although the CA-LPFG performs a comb filter covers C band, except the above three wavelength, the other wavelengths are suppressed to small fluctuations (the four wavelengths laser output is also observed in the experiment but not stable to exist), further more, when the polarization situation is changed, the power at a certain wavelength is quite different (as illustrated in Fig. 2(b), the power is −7.112dB, −5.876dB and −4.84dB at 1561.2nm for different SOP), and the power difference causes different cavity loss and distinct mode competition. On the other hand, the single-wavelength and dual-wavelength laser output are much more stable, for the wavelength shift is less than 0.1nm and power change is less than 0.2dB, respectively; the triple-wavelength laser, as illustrated in Fig. 5, with a fluctuation of 3dB, we also ascribe this fluctuation to the mode competition.
The switch of the triple-wavelength by adjusting PC can be qualitatively explained by the spectrum change of CLPFG under different SOP, the wavelength shift and power change in the pass band of the spectrum is considerable, and the anisotropy of each pass band also indicates that the different laser output with different SOP.
The stability of the triple-wavelength laser and dual-wavelength laser is the result of the balance between the PDL in the laser cavity and the mode competition. The CA-LPFG exhibits two main characteristics here: the wavelength-dependent loss and polarization-dependent loss. The wavelength-dependent loss induced by the interference between core mode and cladding mode allows multi-pass band, each pass band could be oscillated if no mode competition existed; moreover, the polarization-dependent loss induced by asymmetric exposure and cascaded enhancement allows certain polarization choice. By adjusting PC, different polarization mode is excited in the cavity, when pass through the CA-LPFG, it not only choose the oscillation laser wavelength but also choose the polarization state of the laser. The choice of SOP affirmatively enhances the PHB and therefore greatly decreases the mode competition. This greatly improves the stability of multiwavelength lasing output at the room temperature.
4. Conclusion
In conclusion, we use the CA-LPFG, whose transmission spectrum is changed with different SOP, as a polarization-depedent multiwavelength filter; and the three equal wavelength spacing MS-EDFL is realized using the CA-LPFG. The laser shows good switch ability with a switch combination of C 1 3, C 2 3 and C 3 3. More important, this paper gives a simple way of using the polarization characteristic of the CA-LPFG to realize the wavelength switchable fiber laser.
Acknowledgments
The authors acknowledge the support from the Science and Technology Committee of Shanghai Municipal under the contracts 05ZR14078/04DZ14001, the National Natural Science Foundation of China under the grants 60577048, and the Program for New Century Excellent Talents in University of China.
References and links
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