It s Our Business to be EXACT

Transcription

1 671 LASER WAVELENGTH METER It s Our Business to be EXACT For laser applications such as high-resolution laser spectroscopy, photo-chemistry, cooling/trapping, and optical remote sensing, wavelength information is critical. However, the exact wavelength of a variety of lasers, such as tunable lasers and laser diodes, is not known a priori. Therefore, it is necessary to measure laser wavelength to ensure the most meaningful experimental results. The best way to determine the absolute wavelength of CW or quasi-cw lasers is with the 671 Laser Wavelength Meter from Bristol Instruments. The 671 system employs optical interferometry to measure absolute wavelength to the highest guaranteed accuracy, providing real-time laser wavelength information guaranteed by continuous calibration with a built-in wavelength standard. The result is the reliable accuracy that is required for the most demanding applications. What s more, this is accomplished with an unprecedented level of reliability and convenience. Absolute wavelength measured to an accuracy as high as ±.1 nm Continuous calibration with a built-in wavelength standard Operation available from 375 nm to 12 µm Simultaneous measurement of total optical power to an accuracy of ± 15% Measurement rate as high as 1 Hz Input power requirement as low as 1 µw Displays wavelength, wavenumber, or frequency Convenient pre-aligned fiber-optic input for visible and NIR wavelengths Free-space aperture with visible tracer beam for IR and MIR wavelengths Straightforward operation with PC using USB or Ethernet interfaces Web-based application available for tablet or smart phone display Easy to integrate into experiment for automatic wavelength reporting and control

2 RELIABLE ACCURACY FOR GREATER CONFIDENCE the model 671B is a lower-priced alternative with a wavelength accuracy of ±.75 parts per million. The MIR version of the model 671B, however, has a wavelength accuracy of ± 1 part per million. Accuracy in Wavelength Units 671B 671B-MIR 671A Accuracy in Frequency Units 671B 671B-MIR 671A Accuracy (pm) Accuracy (MHz) GUARANTEED ACCURACY The most important aspect of a laser wavelength meter is its accuracy. Bristol Instruments guarantees this specification by taking into account all factors that can affect wavelength measurement. Wavelength accuracy is quantified by Bristol Instruments using the NIST definition for expanded uncertainty. Components of error arising from both systematic and random effects are included. Systematic errors result in an offset between the measured value and the true value. Random errors result in measurements that have a statistical distribution associated with short-term measurement repeatability. The 671 system is designed to address both types of uncertainty. Continuous calibration with a built-in wavelength standard corrects for potential sources of systematic error. Random errors are minimized with a robust Michelson interferometer design. Long-term wavelength measurement data of a DFB laser locked to an absorption line of acetylene. The specified accuracy is given by the dashed lines. 2

3 671 SERIES LASER WAVELENGTH METER CONTINUOUS CALIBRATION To achieve the reliable accuracy that is expected from Bristol Instruments, the model 671 is continuously calibrated with a built-in HeNe laser wavelength standard. The HeNe laser is an ideal reference source because its wavelength is well-known and fixed by fundamental atomic structure. To measure wavelength to the highest accuracy of ±.2 parts per million, the 671A system uses a stabilized single-frequency HeNe laser as a wavelength reference. This laser is stabilized by using a very reliable balanced longitudinal mode technique. A standard HeNe laser is used with the model 671B to achieve an accuracy of ±.75 parts per million EXCEPTIONAL REPEATABILITY The design of the 671 Laser Wavelength Meter is based on a rigorous understanding of Michelson interferometer technology and how it is applied to laser wavelength measurement. This results in exceptional measurement repeatability, defined as the standard deviation for a five-minute period after the instrument has reached thermal equilibrium. The repeatability specification for the model 671A is ±.3 parts per million (±.6 parts per million for the IR version). The model 671B has a repeatibility specification of±.1 part per million. This performance ensures that all wavelength measurements are well within the specified accuracy limits. The exceptional repeatability of the 671 Laser Wavelength Meter provides the ability to detect small changes in laser wavelength. That is, the wavelength resolution of the 671 system is approximately two times the specified repeatability. Therefore, the model 671A has a wavelength resolution of about.6 parts per million (.6 pm at 1 nm), and the wavelength resolution of the model 671B is about.2 parts per million (.2 pm at 1 nm). PRECISE INTERFEROMETER TECHNOLOGY Several methods are available to measure laser wavelength. Bristol Instruments uses optical interferometry for the model 671 because such techniques have been proven to be the most precise and reliable for wavelength measurement instrumentation. A Michelson interferometer is used to generate information from the interference of two beams that originate from the same source. The optical input (depicted in blue) is split between a fixed path and a path that is smoothly changing in length. Both beams are reflected and recombined to produce an interference pattern that is a consequence of the changing phase relationship between the beams. The model 671 generates interference patterns simultaneously from the laser under test and from a built-in HeNe reference laser (depicted in red) that is travelling the same optical path but in the opposite direction. By comparing these interference patterns, the wavelength under test can be determined accurately. Michelson Interferometer Schematic 3

4 OPERATION The 671 Laser Wavelength Meter measures absolute laser wavelength with an unsurpassed level of versatility and convenience. Four broad wavelength configurations are available, laser input is straightforward, and the system has a very high sensitivity. Wavelength is calculated using an on-board digital signal processor and then displayed on a PC, tablet, or smart phone. What s more, it is easy to integrate the 671 Laser Wavelength Meter into an experiment for automatic wavelength reporting or control. BROAD WAVELENGTH COVERAGE The model 671 is available in four broad wavelength configurations to satisfy virtually any experimental requirement. These ranges are the VIS ( nm), NIR (52-17 nm), IR (1-5 µm), and MIR ( µm). VIS ( nm) NIR (52-17 nm) IR (1-5 µm) MIR ( µm) CONVENIENT LASER INPUT A laser under test enters the VIS and NIR versions of the model 671 through a pre-aligned FC/PC fiber-optic input connector. This ensures optimum alignment of the laser beam to the instrument s interferometer resulting in uncompromised accuracy. With fiber-optic input, the 671 system can be placed in an out of the way location, thereby conserving valuable optical real-estate. For free beam lasers, Bristol Instruments offers the LC-1 Fiber-Optic Input Coupler, a simple way to launch a laser beam into fiber. Since fiber is not readily available for infrared wavelengths, the laser under test enters the IR and MIR versions of the model 671 through an input aperture. To facilitate alignment to the instrument, the internal HeNe reference laser is emitted from the input aperture as a visible tracer beam. The laser under test is simply superimposed on the tracer beam to optimize alignment. This is accomplished by using the three adjustableheight legs (±.25 ) that are provided. 4

5 HIGH SENSITIVITY The minimum input signal required by the 671 Laser Wavelength Meter is as low as 1 µw. Therefore, only a small portion of the laser under test needs to be diverted from an experiment. 671 SERIES LASER WAVELENGTH METER In addition, the electronic gain of the 671 system is adjusted automatically to accommodate changes in the input signal. This is particularly useful when scanning a tunable laser over its wavelength range. Input power (µw) A Minimum Input Power A-VIS 621A-NIR 621A-IR Input power (µw) B Minimum Input Power B-VIS 621B-NIR 621B-IR 621B-MIR SIMULTANEOUS OPTICAL POWER MEASUREMENT The VIS and NIR versions of the 671 system measure total optical power in order to provide a more complete analysis of laser performance. The absolute accuracy of the power measurement is ± 15%, and is reported in mw or dbm. The IR and MIR versions are not equipped with a power detector and therefore do not provide an absolute power measurement. However, all 671 systems display a graphical representation of the relative intensity to aid with instrument alignment. 5

6 INSTRUMENT INTERFACE The 671 Laser Wavelength Meter calculates wavelength in real-time using an on-board digital signal processor. Therefore, the measured wavelength information can be reported or used in a variety of ways. Data can be transferred to a PC via USB or Ethernet and displayed using a Windows-based software program provided by Bristol Instruments. 671 SERIES LASER WAVELENGTH METER A web-based application can also be used to display wavelength measurements using a tablet or smart phone. In addition, it is easy to integrate the 671 system into an experiment for automatic wavelength reporting and control using SCPI (Standard Commands for Programmable Instruments) for custom or LabVIEW programming. DISPLAY & REPORTING The 671 Laser Wavelength Meter displays the wavelength and power measurements through several available methods. The 671 system connects directly to a PC operating with Windows 7, 8 or 1 using Bristol Instrument s intuitive NuView software. The NuView software connects directly using USB, or over a wireless network using Ethernet. Data can also be transferred to a PC using SCPI for custom or LabVIEW programming. For even more convenience, the 671 system is also accessible by the latest web browsers. This allows the user to access the 671 system using a PC, tablet, or smart phone as a convenient portable display, enabling laser wavelength information to be displayed anywhere in the laboratory. NUVIEW DISPLAY The Wavelength Screen of the NuView software displays the absolute wavelength of the laser under test. Data is reported automatically to verify laser wavelength in real time, or to allow a laser to be set precisely to an important wavelength. The VIS and NIR versions of the 671 system also display the measured optical power of the laser under test. The IR and MIR versions of the 671 system are not equipped with a power detector, and therefore do not provide an absolute power measurement. However, all 671 systems display a graphical representation of the relative intensity to aid with instrument alignment. 6

7 The NuView software features many user customizable features to tailor the measurements to specific applications. Measurement units can be expressed in wavelength (nm or µm), wavenumber (cm -1 ), or frequency (GHz or THz). Power is reported in linear (mw) or logarithmic (dbm) format. 671 SERIES LASER WAVELENGTH METER The displayed wavelength and power values can be updated with each new measurement, or display a rolling average of as many as 1 measurements. The 671 system will collect the required number of measurements before displaying the averaged value, and will update the display with each successive measurement. In addition, wavelength data can be logged to a file using a *.csv format to be used with other graphing programs. Data can be logged by number of measurements, amount of time, or continuously until the user manually stops the logging. The NuView software features an integrated Trend Screen that automatically charts a laser s wavelength over time. The Trend Screen displays a rolling graphical trace of up to 1, wavelength measurements. The Trend Screen also computes a variety of statistics over the measurement period. These include the maximum and minimum wavelength measurements, laser drift (current wavelength - start wavelength), standard deviation, and the mean. These values are reported in a table below the graph. Measurement data of a temperature-stabilized DFB laser as the cavity temperature increases by.1 C increments. The Trend Screen highlights the oscillations caused by the laser s control algorithim, as well as the overall steps in wavelength. TABLET OR SMART PHONE DISPLAY The web-based display application allows remote connection to the 671 Laser Wavelength Meter to easily provide a portable wavelength measurement display. A 671 system with a wired connection to a router can be accessed by any wireless capable computer, tablet, or smart phone connected to the same network. The display application features customizable display features similar to the NuView software. Measurement units can be expressed in wavelength (nm or µm), wavenumber (cm -1 ), or frequency (GHz or THz). Power is reported in linear (mw) or logarithmic (dbm) format. The displayed wavelength and power values can be updated with each new measurement, or a rolling average of as many as 1 measurements can be reported. The 671 system will collect the required number of measurement before displaying the averaged value, and will update the display with each successive scan. 7