Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2002-04-10
2004-11-02
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S122000, C359S341100, C359S341300, C372S006000, C372S022000
Reexamination Certificate
active
06813429
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sources of optical pulses and methods of generating optical pulses, particularly tunable pulses, by use of the soliton-self-frequency shifting effect in an optical amplifier based on doped holey fiber.
2. Description of the Related Art
Wavelength tunable ultrashort (femtosecond and picosecond duration) optical pulse sources have applications in areas as diverse as ultrafast spectroscopy, materials processing, optoelectronics, nonlinear optics and optical chemistry. Traditionally, femtosecond (fs) pulse sources have been based on bulk crystal materials (most commonly Ti:sapphire), and have employed passive mode-locking techniques, such as Kerr-lens mode-locking, that make use of fast intracavity saturable absorber effects. Whilst excellent performance characteristics have been achieved, and successful commercial products and application areas have been developed, these traditional sources offer a limited range of directly accessible wavelengths and continuous broadband tuning ranges, particularly above 1.1 &mgr;m. In general, extending this femtosecond technology to obtain broader tuning ranges and longer wavelengths requires the use of bulk parametric nonlinear devices such as optical parametric oscillators (OPOs), generators (OPGs), or amplifiers (OPAs), pumped by a bulk femtosecond laser. Such devices add to the complexity and cost, and increase the physical size of the overall system. Moreover, bulk crystal lasers require high-precision alignment and are often pumped by expensive, high-maintenance gas lasers. An alternative approach to obtain broadband tunability is to first generate a broadband supercontinuum spectrum and to spectrally filter out (pulsed) radiation at the desired wavelength (or wavelengths). This technique is commonly referred to as spectral slicing. A supercontinuum spectrum itself (unfiltered) also has many applications including metrology and optical coherence tomography and convenient/practical means to generate such broadband spectra are required. For many applications the spatial mode quality of the supercontinuum beam is an important issue and generally a high quality mode (e.g. a single transverse mode) across all wavelengths is required.
An attractive way of achieving tunability from an ultrashort pulse system is to use the soliton-self-frequency shift (SSFS) effect. The discovery of the SSFS effect in optical fibers was first reported in 1985-1986, and opened up the exciting possibility of obtaining widely wavelength tuneable femtosecond soliton pulses from a variety of optical sources, including fiber-based sources. Femtosecond pulses launched in a suitable optical fiber will propagate as solitons, and Raman frequency shifting within the spectra of the individual solitons gradually alters the wavelength of the pulses. The amount of alteration, or tuning, is governed by factors including pulse power, fiber material and fiber length.
Optical fiber requires certain characteristics to support the SSFS effect. A sufficient level of optical nonlinearity is required to enable solitons to develop and propagate. The nonlinearity experienced by a pulse depends on the amount of energy in the pulse, so pulses propagating in the fiber therefore need to have sufficient energy for soliton formation. Also, to obtain the self-frequency shift, the fiber needs to have anomalous dispersion over the wavelength range of interest, namely the wavelength of the initial launched pulses, and the required tuning range.
Various practical demonstrations of the SSFS effect have been reported. Nishizawa and Goto [1, 2] have reported SSFS in a standard polarization maintaining fiber, using femtosecond pulses from an erbium-doped fiber laser. A soliton output tunable over 1.56 to 1.78 &mgr;m was achieved. A further device using nJ pulses from an erbium-doped fiber laser and SSFS in a standard silica fiber has been reported by Fermann et al [3]. An alternative arrangement by Liu et al [4] uses a tapered microstructured silica fiber to provide SSFS of femtosecond nJ pulses from a Ti:sapphire-pumped OPO. The tapering and microstructuring of the fiber is used to give a large anomalous dispersion with a flattened profile in the wavelength region of interest, 1.3 to 1.65 &mgr;m. In each case, tuning is provided by varying the power of the pulses launched in the fiber.
However, the use of conventional silica fiber for SSFS has limitations, in that it is only possible to obtain anomalous dispersion for wavelengths beyond ~1.3 &mgr;m. This precludes the use of SSFS for achieving tunability in the desirable but difficult to access wavelength region of 1 to 1.3 &mgr;m. Also, the anomalous dispersion and nonlinearity available may be less than optimal for any particular tuning range of interest.
Furthermore, as mentioned, relatively high pulse energies are required to exploit the fiber nonlinearity sufficiently to achieve soliton propagation. Typically, nJ pulses have been used in the prior art. This requirement puts limitations on the laser sources which can be used to drive the SSFS effect. Also, this may have an adverse effect on the available tuning, because tuning is typically achieved by varying the pulse energy/power.
BRIEF SUMMARY OF THE INVENTION
The present invention seeks to address the limitations of the prior art by providing a more versatile tunable ultrashort optical pulse source based on the SSFS effect. This is achieved by using, as the SSFS medium, in one embodiment, a holey fiber having a doped core and configured as an amplifier. The unusual properties of holey fibers, and also, to some extent, tapered fibers, can be exploited to provide a fiber with anomalous dispersion at virtually any desired wavelength, so that it is possible to access wavelength regions not attainable with conventional fibers, including 1 to 1.3 &mgr;m and below. Also, such fibers can be tailored to have a much greater nonlinearity than conventional fibers, so that lower pulse energies can be used to achieve solitonic operation. The present invention can be readily utilised using pJ pulse energies.
The ability to use lower pulse energies is further enhanced by configuring the fiber as an amplifier. Providing internal amplification in this way allows the use of low energy pulses which are then amplified within the fiber until they have sufficient energy to experience the nonlinearity of the fiber, propagate as solitons, and then undergo SSFS. Thus, a wide range of pulsed sources are suitable for use in the present invention, given the minimal limitations on both power and wavelength. For example, wavelength shifting of relatively low energy pulses directly from a simple diode-pumped fiber oscillator is possible. Moreover, the use of an amplifier allows the tuning of the SSFS to be achieved by varying the power of the amplifier pump source, instead of the prior art method of varying the output power of the pulse source. This decoupling of the wavelength tuning from the operation of the pulse source can be advantageous in practice since the fundamental pulse source is left running and does not need to be adjusted at all to effect wavelength tuning of the system output Furthermore, the distributed amplification process offers tuning over a broader frequency range than has hitherto been possible with the passive SSFS devices of the prior art; an embodiment of the present invention has produced femtosecond pulses at wavelengths as long as 1.58 &mgr;m from pulses having a wavelength of 1.06 &mgr;m, corresponding to a frequency shift of 69 THz, which is one third the frequency of the input pulses. The source configuration can also be used to generate pulsed output with an ultra-broadband optical spectrum. The generation of this ultra-broad optical spectrum relies upon supercontinuum generation/effects within these highly nonlinear fibers.
Accordingly, a first aspect of the present invention is directed to a source of optical pulses, comprising: an optical source operable to generate ultrasho
Furasawa Kentaro
Monro Tanya
Price Jonathan Hugh Vaughan
Richardson David John
Stallman & Pollock LLP
Ullah Akm Enayet
University of Southampton
Wood Kevin S.
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