Measurement of an optical amplifier parameter with polarization

In an embodiment for obtaining accurate noise figure measurements for any degree of saturation of an optical amplifier, a polarizer is located at the output of the optical amplifier. The amplified spontaneous noise (ASE) produced by an optical amplifier is not polarized, whereas the amplified signal has a well defined state of polarization which is preferably linear. If the amplified signal is not linearly polarized, it can be rendered linearly polarized in one direction by means of a polarization controller located downstream of the polarizer. By setting the polarizer to have its state of polarization orthogonal to that of the linearly polarized amplified signal, the spectral density of the ASE from the polarizer can be measured without associated distortion due to the signal. By sequentially adjusting the polarization controller to minimize and then maximize the signal which it passes, sequential measurements of the ASE spectral density and gain of the optical amplifier can be obtained. Continuous measurements of the ASE can be obtained by placing a splitter such as a 3dB coupler between the polarizer and the optical amplifier and setting the state of polarization of the polarizer to be continuously orthogonal to the state of polarization of the amplified signal from one leg of the splitter. At the same time, a signal from the other leg of the splitter which consists of ASE noise plus the amplified signal can be used to obtain the gain of the optical amplifier. From the measurements of the ASE with and without the amplified signal, the noise figure of the optical amplifier can be calculated.

FIELD OF THE INVENTION 
This invention relates generally to the testing of optical amplifiers. More 
particularly, this invention relates to the measuring of the noise 
characteristics of optical amplifiers under various operating conditions. 
BACKGROUND OF THE INVENTION 
The primary parameters for characterizing the performance of an optical 
amplifier are the gain, the output power and the noise figure. The gain 
and output power of an amplifier are relatively easy to measure. However, 
the accurate determination of the noise figure is more difficult, 
particularly in the case of saturated operation, which is of great 
practical importance. The problem arises when the amplified spontaneous 
emission (ASE) spectral density is measured at the signal wavelength in 
the presence of a large input signal. A common technique for measuring the 
noise figure of, for example, an erbium doped fiber amplifier involves 
fitting a curve to the ASE level near the signal and then extrapolating 
the curve to find the ASE noise level at the signal wavelength. The major 
problem with this technique is that the ASE spectrum is distorted by the 
sidebands of the signal source and by the optical spectrum analyzer 
response in the presence of a strong signal. 
It is an object of this invention to provide a method and apparatus for 
more accurately measuring the noise figure of an optical amplifier under 
various operating conditions. 
SUMMARY OF THE INVENTION 
This object is achieved by using the polarization properties of the signal 
and the noise to controllably isolate the signal from the noise. In an 
optical fiber amplifier, the amplified spontaneous emission noise 
generated is not polarized; and, the input signal is polarized in one 
direction. By locating a polarizer at the output of the optical amplifier, 
and setting the polarizer to a state of polarization which is orthogonal 
to that of the amplified signal, the amplified spontaneous emission noise 
can be obtained and measured without the associated distortion due to the 
amplified signal. Using the value obtained for the amplified spontaneous 
emission and the gain of the optical amplifier, the noise figure (NF) can 
be calculated. In one embodiment, a polarization controller followed by a 
polarizer is placed between the optical amplifier and an optical spectrum 
analyzer. By either alternating the polarizer to pass and block the 
amplified signal or by alternately including and excluding the polarizer 
from the optical path, sequential measurements can be obtained of the 
amplified spontaneous emission without the amplified signal being present, 
and of the amplified spontaneous emission with the amplified signal; the 
latter being used to obtain the gain of the optical amplifier. In another 
embodiment, by using a splitter such as a 3dB coupler to split the signal 
from the optical amplifier into two parts and directing one part of the 
split signal to a polarizer set to be orthogonal to the polarization of 
the amplifier signal, and the other part of the split signal directly to a 
detector, simultaneous measurements can be made of the amplified 
spontaneous emission without the amplified signal being present and of the 
amplified spontaneous emission with the amplified signal being present, 
the latter being used to obtain the gain of the optical amplifier.

DETAILED DESCRIPTION 
Erbium-doped fiber amplifiers are extremely attractive components for 
modern lightwave systems. Their attractive features include high 
efficiency, high output powers, polarization insensitivity and the ability 
to operate with noise figures near the 3dB quantum limit. Most current 
studies on the noise characteristics of erbium doped fiber amplifiers have 
focused on their behavior in the small-signal regime applicable to 
pre-amplifiers, where low noise is of paramount importance. However, 
another major application of erbium doped fiber amplifiers is as in-line 
amplifiers for long-haul transmission where both low noise and high output 
powers are required. Such amplifiers will be operated under a moderate 
degree of saturation. A final application of erbium-doped fiber amplifiers 
is as power amplifiers, where the output power is of primary importance, 
although low noise is also a desirable characteristic. 
As noted above, the noise figure is a difficult parameter to determine, 
particularly when the amplified spontaneous emission (ASE) noise level is 
measured in the presence of a large input signal. There is here disclosed 
a method and apparatus for overcoming this problem by means of 
polarization nulling. 
This invention is based on the fact that the amplified spontaneous emission 
noise produced by the optical amplifier is randomly polarized, whereas the 
amplified signal is polarized in one direction. Thus, by locating a 
polarizer at the output of the optical amplifier and setting the state of 
polarization of the polarizer to be orthogonal to that of the amplified 
signal, the ASE without the distortion due to the amplified signal can be 
obtained and measured. The NF can be accurately determined using this 
measured value and the gain of the optical amplifier. 
Referring to FIG. 1, there is illustrated an arrangement for obtaining the 
noise figure of an optical fiber amplifier using polarization-nulling. A 
signal source which lies within the gain bandwidth of the optical 
amplifier such as, for example, 1.554 .mu.m generated by a laser diode is 
passed through an optical isolator 20, a 1 nm bandwidth bandpass filter 
22, an attenuator 24, a 90%:10% fused fiber coupler 26 and a second 
isolator 28. The input signal is monitored at the 10% port of the fused 
fiber coupler 26 by a power detector 30 connected to a Hewlett-Packard 
(HP)8153A lightwave multimeter to determine its value. A source of pump 
power such as, for example, a 980 nm signal generated by a Ti:sapphire 
laser is connected to the 90% port of a 90%:10% fused fiber coupler 32, 
and is measured at the 10% port by a power detector 34 connected to an 
HP8153A lightwave multimeter. The pump signal from the coupler 32 and the 
input signal from the second isolator 28 are combined using a JDS Fitel 
Wavelength Division Multiplexer (WDM) 36. The combined signal from the WDM 
36 is connected to an erbium-doped optical fiber amplifier 38. At the 
output of the erbium-doped optical amplifier, the transmitted pump power 
is separated from the signal by a JDS Fitel WDM 40. The pump power is 
measured by a power detector 42 connected to an HP8153A multimeter. 
The amplified spontaneous emission and the amplified input signal from the 
erbium doped fiber amplifier 38 passes through the wavelength division 
multiplexer 40 and through an optical isolator 44. The filtered amplifier 
output signal from isolator 44 is split into two separate signals by a 
50%:50% fused fiber coupler 46 to permit accurate determination of both 
the amplified signal power and the amplified spontaneous emission power 
under various operating conditions from small signal through saturated 
conditions. One signal from the 3dB coupler 46 is passed through a 1 nm 
bandwidth bandpass filter 48, and is thereafter split by a 90%:10% coupler 
50. The bandpass filter 48 is used to select the signal wavelength and 
reject most of the ASE power. The amplified signal is measured at the 90% 
leg by a power detector 52 connected to an HP8153A meter. The output 
signal at the 10% leg can be used to examine the spectrum passed by the 
bandpass filter 48. 
The other signal from the 3dB coupler 46 is passed through a polarization 
controller 60 which is adjusted to insure linear polarization of the 
amplified signal and then through a polarizer 54 which is set to be 
orthogonal to the state of polarization of the amplified signal. In those 
instances where the amplified signal is polarized in one direction, for 
example linearly, the polarization controller may not be necessary. The 
polarizer 54 suppresses the amplified signal by about 40dB and the ASE by 
about 3dB. This minimizes distortion of the measured amplified spontaneous 
emission spectrum which arises from the source side bands and amplified 
spontaneous emissions as well as from an optical spectrum analyzer which 
may be connected to analyze the signal when the signal is strong. The 
light passed by the polarizer 54 is detected by an Advantest Q8381 optical 
spectrum analyzer 56. The ASE level at the signal wavelength can be 
determined by fitting polynomial to the spectrum recorded by the optical 
spectrum analyzer 56. The measuring of the gain and noise figure of the 
saturated erbium-doped fiber amplifier can be automated by controlling the 
attenuator, lightwave detectors and spectrum analyzer with a computer over 
their GPIB interfaces using an appropriate program. 
The gain, G, of the amplifier can be determined from the powers measured by 
power heads 30, 52. The noise figure (NF) of the amplifier is determined 
by the expression 
##EQU1## 
where P.sub.ASE is the measured ASE noise level in a given bandwidth B, h 
is Planck's constant, v is the optical frequency, and G is the gain of the 
amplifier. 
Using the invention here disclosed, the dependence of the measured noise 
figure on signal power was investigated as a function of suppression of 
the amplified signal. These results are illustrated in FIG. 2 for 0dB, 
5dB, 10dB, 10dB and 40dB suppression of the signal. From FIG. 2 it can be 
seen that 20dB of signal suppression provides an accurate value for the 
noise figure under heavily saturated conditions. The polarization drift is 
sufficiently small that this degree of suppression is maintained for 
extended periods. Thus, extended series of measurements can be carried out 
without any need to readjust the polarizer or polarization controllers. 
Thus, there is disclosed an arrangement which can be automated for 
simultaneously, accurately determining the gain and noise figure of an 
optical fiber amplifier in both the unsaturated and saturated states which 
is simple, inexpensive and easily automated to allow for the rapid 
acquisition of data. 
In those instances where it is not desired that simultaneous measurements 
be obtained of the ASE plus the signal and the ASE absent the signal, the 
3dB coupler 46 can be eliminated. The ASE noise level is measured by 
adjusting the polarization controller to minimize the signal for 
determination of the ASE spectral density and then rotating the polarizer 
90.degree. to pass the signal for the gain determination. 
Using the invention here disclosed, it has been observed that, for small 
input signals, the signal sidebands are well below the ASE noise level and 
the prior art curve-fitting method of measuring ASE noise level is 
relatively accurate. But, as the input signal increases, the sidebands of 
the signal are no longer small compared to the ASE noise level. Thus, with 
large input signals, the sidebands of the signal becomes difficult to 
separate from the ASE level, and these sidebands distort the shape of the 
ASE spectrum around the signal wavelength which result in inaccurate 
determination of the noise figure. The invention here disclosed provides a 
more accurate measurement of noise level because, with 
polarization-nulling, this distortion is eliminated. 
Obviously, the invention here disclosed can be used for measurements of 
counter-propagating pumping and bi-directional pumping to obtain accurate 
operating parameters, such as for example noise figure, for any degree of 
saturation of an optical fiber amplifier. 
It will thus be appreciated that those skilled in the art will be able to 
devise numerous arrangements which, although not explicitly shown or 
described herein, embody the principles of the inventions. Accordingly, 
all such alternatives, modifications and variations which fall within the 
spirit and broad scope of the appended claims will be embraced by the 
principles of the invention.