Light generation method and light source

Details Light generation method and light source Light generation method and light source

2000-01-12

2002-03-19

Hellner, Mark (Department: 3662)

Optical: systems and elements

Optical amplifier

C359S199200

Reexamination Certificate

active

06359724

ABSTRACT:

This application is based on Japanese Patent Application Nos. 11-7724 (1999) filed Jan. 14, 1999, 11-141955 (1999) filed May 21, 1999, and 11-361728 filed Dec. 20, 1999, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light generation method and a light source for generating a single-mode incoherent light having a low intensity noise and a small spectral bandwidth, and more particularly, to a light generation method and a light source for using a wavelength-tunable optical filter to output a single-mode light having wavelength components in a particular band of a white-light band by obtaining this single-mode light from a white-light having wavelength components over a wide-band in a wavelength domain.
2. Description of the Related Art
Single-mode light sources are configured to obtain a single-mode light by using an optical filter to spectrum-slice a white-light having an emission spectrum spreading over a wide-band in a wavelength domain. The single-mode light refers to a light showing a uni-modal spectrum distribution around a particular wavelength.
In addition, the white-light refers to a light having continuous spectral components over a wide-band in a wavelength domain and is also referred to as a Gauss light.
A conventional single-mode light source of this kind is typically comprised of a white-light source
81
and an optical filter
90
as shown in
FIG. 21
, and also has an isolator
82
located in an output section of the white-light source
81
for preventing an unwanted light returning from the optical filter
90
. That is, such a light source is comprised of the wide-band white-light source
81
for generating a wide-band white-light, the wavelength-tunable optical filter
90
having a particular transmission band, and the isolator
82
for preventing an unwanted light returning from the wavelength-tunable optical filter
90
so that a white-light from the wide-band white-light source
81
is filtered when it passes through the wavelength-tunable optical filter
90
via the isolator
82
.
The white-light source
81
may be comprised of an incandescent lamp, a super luminescent diode (SLD), or an amplified spontaneous emission (ASE) generated from an optical amplifier. The optical filter
90
may be comprised of a dielectric multilayer film filter, an acoustooptical filter, or a grating monochromator.
A white-light from the white-light source
81
has wavelength components over a wide-band in a wavelength domain. The single-mode light source for obtaining a single-mode light by spectrum-slicing a white-light using the wavelength-tunable optical filter is a mode-hop-free light source that replaces a wavelength-tunable single-mode laser light source, and is conventionally used not only for optical measurements but also as a simple light source for telecommunications systems based on wavelength-division multiplexing (WDH). The spectrum slicing refers to transmitting a white-light through the wavelength-tunable optical filter to obtain a single-mode light having wavelength components in a particular narrow band of the white-light band.
FIG. 22
shows a mechanism for obtaining a single-mode light by using a filter to spectrum-slice an arbitrary center transmission wavelength of a wide-band white-light. As shown in this figure, the spectral shape of a sliced single-mode light reflects a transmission wavelength characteristic of the filter, but the use of an optical filter having a tunable transmission wavelength enables the center transmission wavelength to be controlled using only the optical filter.
In addition, some single-mode light sources are comprised of a combination of a white-light source and an optical filter to spectrum-slice a single-mode light of a selected wavelength from a wide-band white-light. The wide-band white-light source may be comprised, for example, of an amplified spontaneous emission (ASE) generated from an optical fiber amplifier typically including an erbium-doped fiber amplifier (EDFA). Since a spectrum of an ASE from an optical fiber amplifier generally has no fine structure, a single-mode light can be obtained which has an arbitrary center transmission wavelength &lgr;c selected by the optical filter. In addition, an arrayed waveguide grating (AWG) filter can be used to simultaneously obtain single-mode lights of a plurality of wavelengths.
The conventional single-mode light sources, however, have the following problems: since the optical filter filters a white-light occurring in a wide wavelength domain, the output of the resulting single-mode light is very small. Furthermore, the minimum value of the wavelength spectral bandwidth of the single-mode light obtained and the extinction ratio of lights generated in the overall wavelength spectrum except for its portion corresponding to a center transmission wavelength are limited by the performance of the optical filter used. In addition, since an emission phenomenon in the wavelength domain of a light transmitted through the optical filter is a probabilistic event in terms of the emission in the overall wavelength spectrum, the single-mode light obtained has intensity noise that is likely to increase with decreasing transmission wavelength spectral bandwidth of the optical filter.
That is, the wide-band white-light source
81
of the conventional single-mode light source is comprised of a SLD or an ermium-doped optical fiber amplifier (EDFA) which provides high outputs. If, however, a white-light from such a light source is spectrum-sliced, the output of the resulting single-mode light is very small. If, for example, a white-light uniformly output at 10 mW over a 100-nm band is spectrum-sliced at a bandwidth of 0.1 nm, the output of the resulting single-mode light is 10 &mgr;W at most.
Thus, an attempt is made to amplify such a faint single-mode light using an optical amplifier, but simple amplification does not induce a sufficient emission and a spontaneous emission amplified by the optical amplifier occurs in a band around the single-mode light, thereby significantly degrading the spectral purity of the single-mode light. Such degradation causes the signal-to-noise ratio in both optical communication and measurements systems.
For the optical communications systems based on the WDM technique of multiplexing signals into different wavelengths in the wavelength domain, a light source has been desired to have a low intensity noise and a high spectral purity sufficient to restrain wavelength components other than those of the signal light, in order to prevent the signal-to-noise ratio from being degraded
In addition, the conventional single-mode light source for spectrum-slicing a white-light slices a narrow-band single-mode light from a wide-band light source, so that it has an inherent intensity noise within a short observation period as shown below.
If arbitrary beams are observed over a definite period of time (T), the probability P
T
(m) of finding (m) photons in this period is expressed by the following equation:
P
(
m
)=∫
0
∞
p
(
m,
&ngr;)
W
(&ngr;)
d&ngr;
  (1)
where p(m, &ngr;) denotes a probability density function for the probability of finding (m) photons in an independent population having an average photon flow rate (&ngr;) and W(&ngr;) denotes a probability density function for the average photon flow rate (&ngr;). The population means photons that belong to an identical emission phenomenon in a ring. Counting statistics for such a population conforms to the Poisson distribution, so that the following equation is established.
p
(
m,
&ngr;)=(&ngr;
m/m
!)exp(−&ngr;)  (2)
A chaotic light source such as a wide-band light source is a class of such identical populations each of which meets the poisson distribution in equation (2). However, in photon counting statistics limiting the wavelength band, the probability density function W(&ngr;) for the average photo flow rate (&ngr;) of all populations attenuates as shown by the foll

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