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Faraday Rotators & Optical Isolators Technical Notes

Faraday Rotators and Optical Isolators

I.  Introduction

The negative effects of optical feedback on laser oscillators and laser diodes have long been known. Problems include frequency instability, relaxation oscillations, amplified stimulated emission, and in some cases, optical damage. As lasers have improved, the need to protect laser oscillators and laser diodes from optical feedback has increased. An optical isolator’s ability to allow light to pass unimpeded in one direction, while strongly attenuating light traveling in the opposite direction, eliminates the negative effects of optical feedback. Figures 1 and 2 demonstrate this fact. Figure 1 illustrates noise emission displaced 60kHz from the actual laser frequency of a mode-locked laser due to reflections from an uncoated glass surface. The noise emissions correspond to relaxation oscillations in the laser being driven by residual feedback. Figure 2 illustrates the effectiveness of EOT's Optical Isolators in eliminating this problem.

                       

Figure 1                                                                      Figure 2

At the heart of an optical isolator is a Faraday rotator. Faraday rotators utilize high strength, rare earth permanent magnets in conjunction with a high damage threshold optical element to provide a uniform 45° polarization rotation to light passing through the device. The amount of polarization rotation is, θ = V∫Hzdz where:

           θ = polarization rotation angle
           V = Verdet constant of optical element
           Hz = longitudinal component of magnetic intensity vector
           z = length of optical length of optical element

II.  Operation of an Optical Isolator

An optical isolator consists of three main components, an input polarizer, a Faraday rotator and an output polarizer. As shown in Figure 3, light traveling in the forward direction passes through the input polarizer and becomes polarized in the vertical plane. Upon passing through the Faraday rotator, the plane of polarization will have been rotated 45° on axis. The output polarizer, which has been aligned 45° relative to the input polarizer will allow the light to pass unimpeded. As Figure 4 shows, light traveling in the reverse direction will pass through the output polarizer and become polarized at 45°. The light will then pass through the Faraday rotator and experience an additional 45° of nonreciprocal rotation. The light is now polarized in the horizontal plane and will be rejected by the input polarizer which only allows light polarized in the vertical plane to pass unimpeded.

Figure 3

 

An Optical isolator’s ability to provide nonreciprocal rotation while maintaining a linear polarization is what differentiates it from a λ/4 plate-polarizer type isolator, and allows it to provide higher isolation.

Figure 4

III.  Broadband Optical Isolators for Ti:Sapphire and Other Tunable Lasers

Some applications, such as isolating individual amplifiers in a Ti:Sapphire amplifier chain while maintaining the ability to tune rapidly over the amplifier bandwidth or isolating a femtosecond oscillator from a Ti:Sapphire regenerative amplifier, require optical isolators that are wavelength independent. Unfortunately, standard optical isolators are only capable of providing high isolation and transmission over a narrow range of wavelengths, usually about 30-40nm. For this reason we make broadband optical isolators. Broadband optical isolators are unique in that they are passive devices which provide high isolation and good transmission over a 250nm range simultaneously. Other means can be used to increase an optical isolator’s useable wavelength range, but they require manual adjustments such as translating the Faraday rotator optic within the magnetic field or adjusting the polarizers to compensate for the change in rotation angle. It is important to note that while these adjustments will increase an optical isolator’s useable wavelength range, their effective bandwidth still remains ±15nm at any point in time. Figure 5 shows the transmission and isolation of our broadband optical isolators centered at 800nm. As these curves demonstrate, it is possible to construct an optical Isolator that can provide >30dB isolation and >70% transmission simultaneously over a very large wavelength range.

Figure 5

 

IV.  Operation of Broadband Optical Isolators

A broadband optical isolator achieves its wide bandwidth by compensating for the dispersion in the Faraday rotator optic. While the direction of polarization rotation in a Faraday rotator is dependent upon the direction of the rotator’s magnetic field, the direction of rotation in a crystal quartz rotator is dependent upon the direction of light propagating through it. By using a 45° crystal quartz optical rotator with its dispersion similar to the optic in the Faraday rotator, and aligning the Faraday rotator and crystal quartz rotator such that they rotate the polarization of back reflected light in opposite directions, the optical isolator becomes less wavelength-dependent. If 45° rotation at the center wavelength is used for both the Faraday rotator and crystal quartz rotator having the same dispersion, the net rotation will be 0° in the reverse direction and 90° (at the center wavelength only) in the forward direction. Figure 6 shows the effect of light traveling through a broadband isolator in the forward direction.

Figure 6

Figure 7 shows the effect of light traveling in the reverse direction through a broadband optical isolator.

 

Figure 7

 

V.  Dispersion in Ultrafast Ti:Sapphire Lasers; Sellmeiers Equation for TGG

In femtosecond Ti:Sapphire lasers, pulse broadening caused by dispersion in the Faraday rotator optic is a major concern. Below is the Sellmeiers Equation for TGG:

References:

1. U. Schlarb and B. Sugg, Phys. Stat. Sol. (b) 182 K91 (1994)

VI.  Key Optical Components of an Optical Isolator

A.    Faraday Rotator optic: The most important optical element in an optical isolator is the Faraday rotator optic. The characteristics that one looks for in a Faraday rotator optic include a high Verdet constant, low absorption coefficient, low non-linear refractive index and high damage threshold. Also, to prevent self-focusing and other thermal-related effects, the optic should be as short as possible. The most commonly used material used for the 650-1100nm range is terbium gallium garnet (TGG). However, terbium doped borosilicate glass can be used in certain customer products.

B.     Polarizers: Also of critical importance in determining the performance of an optical isolator are the polarizers. Desirable polarizer characteristics include high damage threshold, high extinction ratios and low transmission losses. EOT's 1010-1080nm optical isolators use optically contacted fused silica cube polarizers. Broadband optical isolators utilize PBS Cubes. Laser diode optical isolators utilize Corning Polarcor™ polarizers or PBS Cubes.

VII.          Optical Component Specifications

A. Faraday Rotator Optic Specifications:

 
Tb:Glass
TGG

Pulsed Damage Threshold at 10ns

2J/cm²
10J/cm²

Absorption Coefficient at 1064nm

<0.005cm-1
<0.0035cm-1

Nonlinear Refractive Index

2.7x10-13esu
8x10-13esu

Index of Refraction at 1064nm

1.720
1.95
Verdet Constant at 1064nm (min/Oe-cm)
0.098

0.125

B. Polarizer Specifications:

 
1010-1080nm Isolators
Broadband Isolators
Low Power Laser
Diode Isolators
Medium Power
Laser Diode Isolators

Material

PBS
PBS
Polarcor™
PBS

Pulsed Damage Threshold at 10ns

10J/cm²
1J/cm²
25W/cm² (CW)
1J/cm2

Refractive Index

1.517 at 1064nm
1.48216 at 800nm
1.529 at 583nm
 

Transmission at 22°C 

>92%
>92%
>82% at 800nm
>88% at 800nm

Isolation at 22°C 

>30dB
>30dB
>30dB
>27dB

 

VIII.    Isolation Defined

The two main factors which determine isolation are the extinction level of the polarizers and rotator optic, and the radial uniformity of the Faraday rotator. The radial uniformity is the angle of rotation across a cross section of the rotator optic. The following equation gives the isolation ratio (I.R.) due to radical uniformity for a Faraday rotator when placed between polarizers having an extinction of ≥10-5 for a Gaussian beam:


I.R. = -10log P Leakage/P Incident
or: I.R. =
Isolation Ratio due to radial uniformity

Where:
a = Clear aperture radius
w = Gaussian beam 1/e2 intensity, radius = 2/3 of clear aperture
βr = Radial variation of rotation = θ(r) -θ on axis

IX.           Pulsed Damage Threshold; Root "T" Scaling

Pulsed damage thresholds for EOT's Faraday rotators can be obtained for various pulsewidths by using the Root “T” Scaling method.

           For 1010-1080nm and broadband Faraday rotators: 10J/cm²(τ/10nsec)1/2
           Where τ = the desired pulsewidth