Measurement of an Optical Amplifier Parameter With Polarization

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.

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.

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.

In the drawing:

FIG. 1 illustrates an example of an arrangement for measuring the gain and/or noise figure of an optical fiber amplifier using the invention;

FIG. 2 illustrates curves of measured gain and noise figures as a function of output power for varying amounts of suppression signal.

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.