Abstract:
Apparatus and method for realizing tubular optical waveguides in glass by femtosecond laser direct writing. Irradiation in glass with focused femtosecond laser pulses leads to decrease of refractive index in the modified region. Tubular optical waveguides of variable mode areas are fabricated by forming the four sides of the modified regions with slit-shaped femtosecond laser pulses, ensuring single mode waveguide with a mode field dimension compatible with direct coupling to single-mode optical fibers.

Description:
CROSS-REFERENCE AND RELATED APPLICATION 
       [0001]    The subject application claims priority on Chinese patent application 201610058876.0 filed on Jan. 28, 2016. The contents and subject matters of the Chinese priority application is incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The present invention generally relates to an apparatus and a method for realizing tubular optical waveguides in glass by femtosecond laser direct writing. 
       DESCRIPTION OF RELATED ART 
       [0003]    Femtosecond laser micro-machining has opened up a new avenue in materials processing due to its unique characteristics of ultra short pulse width and extremely high peak intensity. It provides a new approach to fabricate complicate and large scale 3-dimensional micro structures in transparent materials. The femtosecond laser pulses may change the property of the materials in the processed region by nonlinear optical processes, which may be used in high-quality micro- and nano fabrication of a variety of materials. After the irradiation of ultrafast laser pulses, the material undergoes a localized permanent change in the refractive index, which enables the fabrication of optical waveguides, beam splitter and coupler, multimode interferometer, and other photonic devices in the transparent materials. Due to the advantages such as simple fabrication process, high integration, easy to test, and excellent performance, femtosecond laser micro-machining becomes a promising tool in integrated optics, information optics, and quantum optics. 
         [0004]    Currently, two approaches are used to form the optical waveguide by femtosecond laser micro machining. In many glasses such as fused silica and borosilicate glasses, waveguides written by focused femtosecond laser beams consist of cores with increased refractive index that overlap the center of focal spots. Femtosecond laser may induce negative refractive index change in several kinds of glass and crystals, such as ZBLAN glass and lithium niobate; in these materials, waveguides are generally produced by forming a cladding region of reduced refractive index by irradiation of femtosecond laser pulses. The waveguides fabricated in such a way are usually termed “tubular waveguides” as the cladding appears like a tube which surrounds the waveguiding area unaffected by the laser irradiation. The latter approach has gained great attention due to the potential of fabricating large scale optical waveguide. However, in the scheme, the formation of claddings requires a considerable number of scan, and the formed claddings tend to be thick and may cause considerable difficulty in enhancing the waveguide coupling efficiency. These drawbacks prohibit the waveguides based on such technology to fabricate micro-nano photonic device. 
         [0005]    Alternatively, the tubular waveguides may also be longitudinally formed with the spatial modulated femtosecond laser beam, which allows formation of tubular waveguides with small mode areas and thin cladding layers provided that a tight focusing condition is chosen by use of high-numerical-aperture (high-NA) focal lenses. However, the scheme inherently suffers from lack of flexibility in terms of writing 3D photonic circuits due to the limited working distances of high-NA lenses. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides an apparatus and a method for realizing tubular optical waveguides in glass by femtosecond laser direct write. It is efficient and easy to operate. The tubular optical waveguides fabricated by the method of the present invention has controllable mode area and low loss characteristics. 
         [0007]    The apparatus for realizing tubular optical waveguides in glass by means of a femtosecond laser direct write of the present invention comprises a femtosecond laser system, an attenuator, a shutter, a spatial light modulator (SLM), a first reflector, a first convex lens, a slit, a second reflector, a second convex lens, a dichroic mirror, an objective lens, transparent glass material, a computer-controlled XYZ platform, computer, cold light source, a third reflector, and a CCD. 
         [0008]    In the present invention, the pulse energy of the output beam of the femtosecond laser system may be controlled by the attenuator. Along the direction of the laser pulse emitted by the femtosecond laser system, the attenuator, the shutter, and the spatial light modulator (SLM) are arranged along the optic pathway, and the laser pulses and beam, after the reflection by the spatial light modulator, sequentially passes through the first reflector, the first convex lens, and second reflector, the second convex lens, and the dichroic mirror along the optic pathway. Then, the laser pulses are reflected by the dichroic mirror and focused by the objective lens to the inside of the transparent glass material which is fixed onto a three dimensional XYZ platform. The XYZ platform is connected to the computer. The slit is positioned at the focal plane of the first reflector, which is the Fourier imaging plane of the reflection spot from the SLM. After being reflected by the third reflector, the beam from the cold light source is reflected on the bottom of the transparent glass. After being projected by the transparent glass and focused by the objective lens, the beam is projected into the dichroic mirror. After passing through the dichroic mirror, the beam is received by the CCD and the output of the CCD is connected to the computer. 
         [0009]    The present invention further provides a method for realizing tubular optical waveguides in glass based on the apparatus of the present invention and comprises the steps of: 
         [0010]    (1) Designing a first phase modulation mask and writing the first phase modulation mask into the SLM; 
         [0011]    (2) Fixing the transparent glass on the XYZ platform and focusing the femtosecond laser pulses into the transparent glass by moving the XYZ platform to fabricate enclosed claddings; 
         [0012]    (3) setting an opposite direction of the laser as a Z axis, setting width and length direction of the transparent glass as an X and Y axes; after modulation and filtering, the femtosecond pulses are focused into the transparent glass through the objective lens to form a narrow region of reduced refractive index along the X axis with a width of D; using the position as a start point, moving the XYZ platform along the −Y direction for a distance L to form a flat and narrow region with a length of L and width of D inside the transparent glass having the refractive index is less than that of the transparent glass, and using the region as a bottom cladding of a waveguide; 
         [0013]    (4) designing a second phase modulation mask and writing the second phase modulation mask into the SLM; 
         [0014]    (5) tuning the attenuator to adjust power of the incoming laser pulses, moving the XYZ platform to adjust position of the transparent material, after modulation and filtering, focusing and forming the laser pulses into a narrow region along Z direction with a width of D inside the transparent glass; the region is connected to the left side of the region formed in step (3); moving the XYZ platform along the −Y direction for a distance of L to form a planar laser-modified region that is a left cladding of the waveguide; 
         [0015]    (6) moving the XYZ platform to a second position, after modulation and filtering, focusing the laser pulses and forming a narrow region along Z direction with a width of D inside the transparent glass, and making the region to be connected to the right side of the region formed in step {circle around (3)}; moving the platform along the −Y direction for a distance of L to form a planar laser-modified region that is the right cladding of the waveguide; and (7) moving the XYZ platform back to the start position, designing a first mask and writing the first mask into the SLM, moving the XYZ platform for a distance D along +Z axis, tuning the attenuator to increasing power of the laser pulses and focusing the femtosecond laser pulses, after modulation and filtering, and forming a narrow region inside the transparent glass that is on the plane of the starting position and parallel to the plane of the region formed in step (3); moving the platform for a distance of L along −y direction to form a planar laser-modified region that is a top cladding of the waveguide. 
         [0016]    The present invention are advantageous in the following areas: 
         [0017]    The present invention involves transversely writing the tubular waveguides by focusing slit-shaped and spatial-modulated femtosecond laser beams in glass to form the claddings. The present invention writes pre-set phase grating position into the spatial light modulator to modulate the spatial phase of femtosecond laser pulses, the femtosecond laser pulses, after being modulated, show diffracted light intensity distribution in the Fourier transform plane thereof, and through the slit spatial filtering, a first degree diffraction spot is obtained on the Fourier transform plane. By the above-described femtosecond laser pulse phase modulation processing and spatial filtering, the femtosecond laser pulses go through the objective lens and are focused onto a particular transparent material. The light intensity at the focal point is more evenly and narrowly distributed. Since the interaction of the femtosecond laser and these particular materials may induce reduction of the local refractive index, then, the modulated and filtered femtosecond laser pulses may induce narrow area with decreased refractive index inside the transparent material. The transparent material is driven by the computer-controlled XYZ platform to move so that the femtosecond laser direct writes inside the transparent material to have a layered structure with reduced refractive index. Through spatial Integration and reasonable geometric construction, a plurality of such layered structure may be used as claddings to enclose a channel whose cross-section is square shaped, thus, a square ring waveguide inside the transparent material is achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1( a )  shows mask  1  for the SLM with a central grating region, and the vertical length of the region is 12000 μm and the horizontal length of 120 μm.  FIG. 1( b )  shows mask  1  for the SLM with a central grating region, and the vertical length of the region is 800 μm and the horizontal length of 16000 μm. 
           [0019]      FIG. 2  shows a schematic arrangement of an embodiment of the apparatus of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The following description provides embodiments and is not intended to limit the scope of the present invention. 
         [0021]      FIG. 1( a )  shows mask image  1  for the SLM with a central rectangular grating region. The vertical length of the region is 12000 μm and the horizontal length of 120 μm. The gray value inside the grating region changes from 0 to 210 along the vertical direction with a spatial period of 420 μm, and the value outside the region is set to 0. 
         [0022]      FIG. 1( b )  shows mask image  2  for the SLM with a central rectangular grating region. The vertical length of the region is 800 μm and the horizontal length of 16000 μm. The gray value inside the grating region along the vertical direction with a spatial period of 100 μm, and the value outside the region is also set to 0. The masks modulate the phase of the femtosecond laser pulse by changing the refractive index of the liquid crystal surface of the SLM. The diffraction may be achieved on the Fourier Transform Plane of SLM, and the first order diffraction is filtered out through as lit. Set the opposite direction of the laser as the Z axis, the sides of the transparent material as the X and Y axes. After focusing by the objective lens, the modified areas in the material are narrow regions of reduced refractive index along X direction and Z direction with mask  1  and mask  2  respectively. 
         [0023]      FIG. 2  is a schematic setup for the tubular waveguide fabrication. The output beam of a femtosecond laser  1  with an operation wavelength of 800 nm, a pulse width of ˜40 fs, and a repetition rate of 1 kHz. The pulse energy is controlled by the attenuator  2 , and the shutter  3  is used for blocking the laser pulses. The SLM  4  reflects the laser pulses, and the laser pulses are then reflected by the first reflecting mirror  5  and pass through a 4 f imaging system formed by the first convex lens  6 , the second reflective lens  8 , and the second convex lens  9 , and then reflected by the dichroic mirror  10 . The focal length of the first convex lens  6  and the second convex lens  9  are both 70 cm. In the focal plane of the first convex lens  6 , which is also the Fourier imaging plane of the reflection spot by the spatial light modulator  4 , a slit  7  is placed to have a spatial filtering to obtain a first diffraction spot. After the phase modulation and spatial filtering, the femtosecond laser pulses pass through the microscope objective lens  11  having a NA of 0.8 and are focused onto the inside of the ZBLAN glass material  12 . The ZBLAN glass material  12  has a size of 10×7×3 mm, is placed on the platform  13 , and the platform  13  is controlled by the computer  14 . The cold light source  15  emits light that reaches the bottom of the ZBLAN glass material  12 , the light transmitted is then focused through the microscope objective lens  11 , and passing through the dichroic mirror  10 , is received by the CCD  17 . The image received by the CCD  17  is output to computer  14  for real-time view of the entire process. 
         [0024]    After the optical path is arranged, attenuator  2  is used to adjust the laser incident power to 60 mW.  FIG. 1( a )  shows the image of the phase modulation input mask that is input into the spatial light modulator  4 . Adjusting the height of the three-dimensional platform  13  so that the spot is focused to a position that is 50μm from the upper surface inside the ZBLAN glass material  12 . Subsequently, the optical waveguide cladding ring is obtained through the following four-step operation. 
         [0025]    1. The opposite direction to the direction of the laser transmission is the z direction, The width and length of the ZBLAN glass material  12  are the x and y directions. The current position of a platform is set as the original start point for the movement of the platform. Open the shutter  3 , using computer  14  to control and move platform  13  along the −y direction, moving speed is 20 μm/ s, moving distance is 7 mm, and a cladding having a length of 7 mm, a width of 9.7 μm and relatively small refractive index is formed. Closing the shutter  3 , moving the platform back to the original start point, and processing the layered structure with reduced refractive index as the bottom wall of the annular wall waveguide cladding. 
         [0026]    2. The image of the phase modulation mask as shown in  FIG. 1( b )  is loaded onto the spatial light modulator  4 . The three-dimensional platform  13  is driven back to the original start position, and then adjusted to move to the −z direction for 4 μand to the x direction for 6 μm. Open the shutter  3 , control the attenuator  2  to adjust the laser power to 20 mW, and control the three-dimensional platform  13  to move along −y direction, while maintaining the speed and distance unchanged. When the processing is complete, shutter  3  is closed and the platform  13  returns back to the start point. The left side wall of the annular waveguide cladding is obtained. 
         [0027]    3. The platform  13  is driven back to the start position and then adjusted to move along the −z direction for 4 μm and along the −x direction for 6 μm. Open the shutter  3  and control the three-dimensional platform  13  to move along the −y direction, while maintaining the speed and distance. At the end of the processing, the shutter  3  is closed and the platform  13  moves back to the start point. The right side wall of the annular waveguide cladding is obtained. 
         [0028]    4. The image of the phase modulation mask as shown in  FIG. 1( a )  is loaded onto the spatial light modulator  4 . The platform  13  is driven back to the start position and then adjusted to move along the +z direction for 9.7 μm. Open the shutter  3  and attenuator  2  to adjust the power of the laser to 60 mW. Then, control the three-dimensional platform  13  to move along the −y direction, while maintaining the speed and distance. At the end of the processing, the shutter  3  is closed and the platform  13  moves back to the start point. The upper wall of the annular waveguide cladding is obtained. 
         [0029]    After the above described four scanning, the femtosecond laser pulses interact with the ZBLAN glass material  13  to form the optical waveguide cladding. The cladding forms a rectangular cladding glass material which is the core of the optical waveguide. The annular waveguide formed at the end has a length of 7 mm, and the size of the mode field is about 9.7×9.7 μm. 
         [0030]    Experimental results show that the present invention may induce the formation of low-loss tunable annular optical waveguide in a particular transparent material within certain mode field size range. Optical waveguides having the size of 9.7×9.7 μm and 20×20 μmare obtained. In the optical waveguide having a mode field of 9.7×9.7 μm, the loss measurements value is 0.5 dB/cm; the laser emitted by the helium-neon laser system is coupled to one end of the optical waveguide, and a bright and uniform light spot may be obtained from the other end. In addition, based on the above described technology and principle, an optical waveguide with any length may be realized. The experimental processing for making the optical waveguide having the length of 7 mm takes 18 minutes, thus, the process has high processing efficiency.