Optical waveguides – With optical coupler – Particular coupling structure
Utility Patent
1998-06-17
2001-01-02
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Particular coupling structure
C385S033000, C385S051000, C372S006000
Utility Patent
active
06169832
ABSTRACT:
The present invention relates to superluminescent diodes, and more particularly to a system for generating a wavelength stabilized output from a superluminescent diode using a narrow channel wavelength division multiplexer coupler.
BACKGROUND OF THE INVENTION
Superluminescent diodes (SLD) are candidates for use as the light source in interferometric fiber optic gyroscopes (IFOG), where a relatively broadband, low-coherence, and high optical power fiber-coupled source is desired. The wavelength-operating region of the SLD can be in any of several different wavelength regions, but recent efforts have focused on the 1550 nm region. For effective operation in an IFOG, the light source must also exhibit a relatively stable and predictable optical spectrum over a wide temperature range, typically from −55 to +80° C. IFOG applications require a light source thermal sensitivity on the order of 5 parts per million per degree Celsius (ppm/° C.). The inherent temperature sensitivity of the mean wavelength of a typical SLD is on the order of 400 to 500 ppm/° C., which is too large for it to be used in navigation grade IFOGs even when thermoelectric cooling devices and other temperature compensation components, circuits and techniques are utilized.
A prior art system for generating a wavelength stabilized output signal from an SLD which uses a multilayer dielectric interference filter inserted as one of the output coupling optics of a fully packaged SLD is described with reference to FIG.
1
. The system shown in
FIG. 1
includes a superluminescent diode
1
, which generates an optical output signal S that is collimated by lens
2
and directed through a multilayer interference filter
3
. The filtered optical signal S is focused by lens
4
whereupon it is injected into optical fiber lead
5
. It is to be noted that in FIG.
1
, the various optical mounts and alignment fixtures for supporting the optical components
1
,
2
,
3
,
4
, and
5
are not shown. However, it is to be understood that precision mounts and alignment fixtures are necessary to efficiently interface each of the optical components to one another. SLD
1
is designed and fabricated so that its output spectrum, shown schematically in
FIG. 2
, is somewhat larger than the transmittance passband of the interference filter, shown schematically in FIG.
3
. The interference filter is ideally fabricated so that its passband is centered at the mean wavelength of the SLD emission for a specified target operating temperature. In addition, the materials used for the substrate of filter
3
and for the deposition of the filter layers should be chosen to have the lowest temperature coefficients of refractive index and expansion while still providing the appropriate indices of refraction to obtain the desired spectral transmittance. Under these conditions, the spectral properties, including the wavelength stability, of the combined SLD/filter source are determined primarily by the properties of the filter. In principle, as the temperature changes over the device operating range, the interference filter transmittance function will have a minimal shift in wavelength. The SLD spectrum may shift significantly for the same temperature change, but because the SLD spectrum is wider than the passband of the filter, the output signal remains centered at the center of the filter passband. For the multilayer interference filters to operate properly, a collimated input beam, derived from the SLD, must be incident on the filter. This requires that additional optical components, typically discrete micro-lenses, are used to collect and collimate the output of the SLD, and to focus the collimated light, which has passed through the filter into the single-mode fiber lead. Furthermore, the various components should be anti-reflection coated at the SLD mean wavelength to minimize optical feedback to the SLD. This is the same reason why the filter
3
is mounted at a non-normal angle with respect to the collimated beam S. Because this method requires that several discrete components be precisely aligned with respect to one another, precision mounting/alignment fixtures are required for the components.
The limitations and disadvantages of the above-described system for obtaining a wavelength stabilized SLD output are numerous. First, in order to obtain a sufficiently high transmission over a relatively broad spectral range, the possible choices of materials used to form the individual layers of the interference filter are limited to those whose indices of refraction are within a certain range. Therefore, the materials with the lowest thermal coefficients are in general not suitable for fabrication of the filter. A typical thermal response for the mean wavelength of a broadband (≧10 nm) high transmission (>85%) interference filter operating in the 1550 nm region is in the range 30-40 ppm/° C., which is an order of magnitude too large for IFOG applications. Secondly, the use of discrete optical components with their associated fixtures results in a fully packaged wavelength stabilized SLD that is relatively large and which requires a stable thermal and mechanical environment for proper operation. This means that the fully packaged SLD must include associated thermal and mechanical stabilization components and circuits, not only for the SLD semiconductor chip, but also for the discrete lenses, the filter, the fiber lead, and their associated mechanical mounts and alignment fixtures. Third, feedback power levels as small as 10 ppm can seriously degrade the power and wavelength stability of an SLD, and therefore reflected light feedback into the SLD must be minimized. Minimization of light feedback requires that all of the surfaces of the collimating and focusing optics and the fiber lead flat end face must be anti-reflection coated at the wavelength region of interest.
An example of the deleterious effects of reflection feedback into an SLD is shown in
FIG. 4
, which is the actual output spectrum of an SLD/interference filter combination operating in a wavelength region centered around 1510 nm. The feedback results in an asymmetric spectrum with significant ripple, characteristics, which seriously limit the applicability of such a source. Also, since the portion of the SLD spectrum rejected by the filter is actually reflected, extreme care must be taken to align the filter at an angle to the optical axis of the SLD output beam and the collimating and focusing optics. This requires that the passband center wavelength of the multilayer interference filter when it is operated at the offset angle, as opposed to normal incidence operation, match the center wavelength of the SLD emission spectrum. Matching the center wavelength of the SLD output emission imposes additional design and fabrication conditions upon the filter, which is normally fabricated while monitoring its optical properties at normal incidence. Furthermore, operating the filter at non-normal incidence will skew the transmission function of the filter, introducing asymmetries and structure that will reduce the effectiveness of the light source in several applications. Additionally, the power loss resulting from the transmission of the SLD output light through the various components can be significant, and therefore the effective optical power output of the packaged SLD is reduced. All of these special requirements imposed by the operation of the SLD with a discrete interference filter and the associated optical components result in a packaged SLD with degraded performance and reliability, especially in changing mechanical and thermal environments, and a more costly package.
A need, therefore, exists for a simple, cost-effective system, which generates a wavelength-stabilized output from a superluminescent diode with respect to temperature that does not require expensive and bulky optical components and mounting and alignment fixtures.
SUMMARY OF THE INVENTION
The present invention provides a system for generating a filtered “broadband” light output from a superluminescent diod
Curtis Craig
Fendelman Harvey
Kagan Michael A.
Lipovsky Peter A.
Spyrou Cassandra
LandOfFree
System for generating a wavelength stabilized output from a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System for generating a wavelength stabilized output from a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System for generating a wavelength stabilized output from a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2447611