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Which EXFO power meter is compatible with unconnectorised or bare fibre?
The EXFO IQS-1600 and PM-1600 range of power meters are available with a choice of either standard 1 mm or 3 mm wide-area InGaAs detectors.

A wide-area detector is recommended for use with unconnectorised fibre. When used in conjunction with the EXFO bare fibre adaptor, the IQS-1600W and PM-1600W power meters give accurate and repeatable measurements over all the telecom transmission bands.

The IQS-1600W power meter's modules operate within the IQS-600 mainframes and are available with 1, 2 or 4 detector channels per module. The PM-1600W range of stand-alone power meters is available with either 1 or 2 detector channels.

EXFO Bare Fibre Adaptors can be readily loaded with the unconnectorised fibre and snap into place onto the power meter with a magnetic clamp.

Having read the OZ Optics' brochures, I have some questions. Am I correct in assuming that laser to pigtailed fibre coupling seems to be the most stable option, and which lens should I choose for the best coupling efficiency into single mode fibre?
A pigtail style laser to fibre coupler will be the most stable option as there will be less backreflection and insertion loss if the fibre is permanently attached to the coupler unit. The optic and fibre will be securely locked in place to provide the best long-term stability. A receptacle style unit offers the convenience of a removable fibre but long-term repeated plugging of a fibre connector will eventually wear the receptacle and connector and coupling efficiency may fall off over time.

Receptacle style couplers are usually recommended for systems that are not sensitive to backreflection and where different fibres are often used. They are also recommended for high power applications where fibre ends could be burnt, in which case connectors could easily be repolished or reterminated.

To recommend an appropriate laser to fibre coupler we would need to know the wavelength, laser beam diameter (mm) and the laser beam full divergence angle (mrad). We can then select an appropriate focal length lens for coupling into the selected fibre.

Two main conditions must be met to optimise coupling into an optical fibre: 1) The focused spot size must be less than or equal to the core size of the fibre, and 2) The NA of the focused rays must be less than or equal to the NA (or acceptance cone) of the fibre.

If an appropriate focal length lens can be chosen that meets both of the coupling conditions, then coupling efficiencies of up to 50% can be achieved into single mode fibre and 90% into a multimode fibre.

For single mode fibre coupling we quite often find that out of the coupling lenses available, we can only meet one of the coupling conditions. This is because of a rather large and/or divergent input beam or that an exact focal length lens match cannot be found. This means that either the focused spot will be slightly larger than the core size or the NA of the focused rays will be larger than the acceptance angle of the fibre. Under these conditions, typical coupling efficiencies of between 20-30% can be expected into single mode fibres (assuming that a reasonably close match on focal length can be found).

Out of the four types of coupling lens offered by OZ Optics, I would recommend an aspheric as these tend to be better quality lenses than the GRIN or plano-/bi-convex optics and, if you only need to couple a single wavelength, an achromat lens would not be necessary.

For the highest precision fibre launch I would recommend our Elliot | Martock MDE 510 system using a microscope objective or miniature aspheric lens. Bare fibre or connectorised fibre adapters are available making this the highest precision and most flexible option. Please view the following link for more details.

How do I select a temperature control instrument for my cryogenic application?
Temperature control instruments are widely used in cryogenic applications and we are often asked for advice on which of our six Lake Shore models to choose. All six models perform the same basic function of accurate temperature measurement and PID control and, for many applications, more than one of the controllers would provide a perfectly acceptable solution. For more demanding applications, the feature set of a particular instrument will determine the recommended model.

Perhaps the most important factor will be temperature range. For temperatures below 100 mK, the Model 370 AC resistance bridge offers the necessary high input resistance for NTC RTDs and has 21 low-noise auto-ranging current settings with no significant DC component to contribute to sensor self-heating. Unique noise reduction elements allow the Model 370 to significantly reduce measurement noise and reliably measure resistive materials in environments down to sub-20 mK. With up to 16 channels and closed loop temperature control, the Model 370 is an ideal companion to dilution refrigerators and helium-3 cryostats.

At temperatures of 100 mK and higher, the Model 340 is the most fully-featured of the other five instruments in this range and is compatible with the largest number of sensor types. The 340 has the widest temperature range, operating from 100 mK to 1505 K with appropriate sensor selection and, at 100 W, its primary control loop output is the highest of all the controller models. The 340 has a lower number of auto-ranging excitation current settings than the 370 and is thus not as flexible for ultra-low temperature work, but adds diodes, PTC RTDs, thermocouples and capacitance sensors to the NTC RTDs that are used with the 370, making it an excellent general-purpose instrument.

The 340 can accept up to ten sensor inputs through an expansion card and, if funds allow and ultra-low temperature measurement is not required, this model would be our controller of choice. Even if its more advanced features are not required at the outset, it will provide the means to expand your measurement capabilities in the future without needing to buy another controller.

For situations where the expansion capabilities of the Model 340 are not required, the Model 332 has a broadly similar performance but with a higher minimum temperature of 500 mK. It has a slightly reduced set of auto-ranging excitation current settings and a 50 W primary control loop, but retains most of the features of the 340 at a lower cost.

The Model 331S, 331E and 325 controllers have higher minimum temperature of 1.2 K and do not feature the excitation current auto-ranging of the higher specification models, with only two excitation current settings available in each case. Nevertheless, these three instruments will accommodate most sensors and are widely used in cryogenic applications, albeit with some restrictions on the use of certain types of sensor.

The Model 331E is the economy version of the 331S and does not include the IEEE-488.2 interface found on all other Lake Shore temperature controllers but, as with all other models, does include an RS-232C interface. All controllers except the 331E and 325 have two relays and analogue voltage outputs, and all except the 325 have high and low alarms. Despite being the lowest-cost controller, the 325 nevertheless still supports a remarkable range of sensors and has two sensor inputs and control loops.

There may be many factors important to you in selecting a temperature controller beyond those mentioned above. Our instrument selection guide shows a side-by-side comparison of all six Lake Shore controllers with regard to sensor compatibility, operating temperature range, control capability, display features and interface flexibility.

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