Femtosecond Lasers Fabricate Improved Optical Waveguides
Many laboratories around the world are investigating the use of femtosecond lasers for fabricating optical waveguides. Recently, scientists in a US laboratory employed femtosecond lasers to create waveguides in poly(methyl methacrylate) (PMMA), while European-based researchers have demonstrated that high-quality waveguides can be created in glass with pulses from a simple, diode-pumped, femtosecond laser.
Although Bragg gratings have been created in PMMA with femtosecond pulses, researchers at the University of Central Florida's College of Optics and Photonics in Orlando believe that theirs is the first successful attempt to create waveguides in this important material with ultrashort pulses. They focused 20-nJ, 30-fs pulses from a Ti:sapphire laser into a slab of PMMA with a 103 microscope objective and translated the PMMA parallel to the laser beam. They produced a tubular waveguide with a 25-µm diameter (Figure 1a).
Figure 1. (a) A differential interference contrast microscope image reveals the tubular waveguide written in PMMA with femtosecond pulses. (b) The near-field intensity pattern of 633-nm light emerging from the waveguide shows how light is guided in region II.
They deduced that the three regions (I, II and III in the figure) have different refractive indices and are created by thermal expansion of material heated by the pulses. In region I, the heated material expands, reducing its density and refractive index, while in region II, the material is compressed between the expanding material in region I and the unheated material in region III. Thus, the refractive index in region I is reduced from the normal, while that in region II is increased. (The index in region III is unchanged). Light is guided in region II, which has the highest index. This thesis is supported by the pattern of light that emerges from the end of the waveguide (Figure 1b).
Further support for the thesis was provided by a novel approach to measuring the waveguide's refractive-index profile. The researchers reasoned that, because the densities of the three regions were different, their solubilities for a given solvent also would be different. They treated the end of the waveguide with methyl isobutyl ketone for 60 minutes, then cleaned it and analyzed the etched surface with a white-light interference microscope. They observed that region II had the fastest etch rate, that region III had the next-fastest and that region I had the slowest. They assumed that the PMMA's refractive index was proportional to density, which in turn is proportional to etch rate, and they concluded that the waveguide's index profile was simply the inverse of its spatial etch profile.
Figure 2. (a) A numerical analysis shows the sinusoidal solutions that are the azimuthal modes of the tubular waveguide. (b) This near-field intensity pattern of light emerging from the waveguide was obtained for a particular coupling arrangement of light into the waveguide. (c) The calculated intensity pattern for simultaneous excitation of equal parts of modes ν = 3 and ν = 4 is very close to the observed pattern in b.
They also performed a mathematical analysis of the modes that would propagate in a tubular waveguide like the one they created. They found that, in the radial direction, the solutions are modified Bessel functions and that, because of the narrowness of the tubular waveguide, only a single mode can oscillate in the radial direction. In the azimuthal direction, the solutions are sinusoidal, and multimode oscillation is expected (Figure 2a). In practice, which azimuthal modes oscillate will depend on how light is coupled into the waveguide, and the researchers excited several modes and combinations of modes by altering the coupling parameters (Figures 2b and 2c).
While the scientists in Florida were writing waveguides in new polymer materials, a coalition of researchers from the Istituto di Fotonica e Nanotecnologie-CNR in Milan, Italy, from the Max Planck Institut für Kernphysik in Heidelberg, Germany, and from High Q Laser Production GmbH in Hohenems, Austria, was moving the craft of writing waveguides with femtosecond pulses closer to industrial implementation. The coalition wrote waveguides in erbium-ytterbium-doped phosphate glass with femtosecond pulses from a simple, diode-pumped, unamplified Yb:glass laser.
Most femtosecond waveguide-writing until now has utilized the pulses from an amplified Ti:sapphire laser, which in turn is pumped by a visible laser, so the achievement is a significant simplification. The researchers employed a passively mode-locked Yb:glass laser that was pumped by a 5-W, 976-nm laser diode and cavity-dumped at 166 kHz. The resonator was stretched in a Z-fold configuration to reduce the mode-locking frequency to 22 MHz. The 1040-nm output pulses had a duration of 300 fs, and their average power was 45 mW.
Figure 3. This differential interference contrast microscope image shows the waveguide fabricated in Er-Yb-doped glass by femtosecond pulses from a diode-pumped glass laser.
They focused these pulses with a microscope objective to a depth of 170 µm in the Er-Yb-doped glass sample, and translated the sample perpendicular to the incoming laser beam. The resulting waveguide (Figure 3) could guide light at both 633 nm and 1.55 µm, if the translation speed during the writing process was appropriate.
As an example of propagation at 1.60 µm, they acquired the near-field pattern of the light emerging from the waveguide when a single-mode telecom fiber was butt-coupled to it (Figure 4). When they changed the alignment between the fiber and the waveguide, they observed that the intensity -- but not the shape -- of the light emerging from the waveguide changed, and they concluded that the waveguide is single-mode at 1.60 µm.
Figure 4. The 1.6-µm radiation emerging from the fabricated waveguide shows a Gaussian intensity distribution. The dashed curves are the intensity pattern of the input beam, and the solid curves are the beam that emerges from the waveguide.
For what they believe is the first time in waveguides written by a femtosecond laser, the waveguide mode was nearly perfectly matched to that of a standard telecom fiber, with coupling loss evaluated at 0.18 dB. By measuring the ratio of the input and output powers for the waveguide -- and taking into account the Fresnel and coupling losses -- they estimate the propagation loss to be ~0.7 dB/cm. They also calculated the refractive-index change in the waveguide to be ~0.009.
Figure 5. The 12-mm waveguide showed positive net gain over virtually the entire C-band when pumped with 980-nm radiation from a laser diode.
To measure the gain, the researchers pumped a 12-mm length of waveguide from both directions with a 980-nm laser diode. At a pump level of 250 mW, they observed small-signal internal gain across the C-band, with a maximum of 2.7 dB at 1533 nm (Figure 5). This C-band gain was greater than the waveguide's 1.5-dB insertion loss, and by terminating the waveguide with two fiber Bragg gratings, they observed laser action.
The scientists believe that this is the first demonstration of net gain and laser action in a waveguide written with femtosecond pulses.
- optical waveguide
- Any structure having the ability to guide the flow of radiant energy along a path parallel to its axis and to contain the energy within or adjacent to its surface.
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