Selecting laser combiner

Lasers are the most expensive yet the least reliable component of mesoSPIM, so laser selection should be approached carefully.

:bulb: The mesoSPIM control software does not know anything about the laser hardware, there is no serial communication or other digital feedback collected from the device. The mesoSPIM software simply generates a) TTL digital signal and b) analog signal for each laser line listed in its config file. Digital modulation allows to turn the laser on/off for timely light-sheet generation, while analog modulation allows to adjust the laser power interactively from mesoSPIM control software. It is up to the user how these signals are connected to the laser. Most lasers have both analog and digital inputs, so mesoSPIM is very versatile and can work with laser combiners from various manufacturers. The laser combiner should be set to both digital and analog mode using its own control software, provided by the manufacturer.

The recommended power is 100 mW for each line before the fiber (150 mW for red laser, because some red-shifted dyes are dim), single-mode fiber coupling (so 50% power loss after the fiber). The lasers must be analog and digitally modulated. The most commonly used laser combination is 405, 488, 561, 638 (or 647) nm.

Oxxius

We find that currently the most cost-efficient laser combiner is provided by Oxxius L4Cc (405, 488, 561, 638 nm) with add-on module MDL-FSTM for fast switching between two laser fiber outputs. Although it is relatively (!) affordable, we and some other mesoSPIM owners observe quality issues with 561 and 638 laser lines, whereby the laser spontaneously overheats and turns off. This typically requires sending the laser combiner for repair.

TOPTICA

From our experience, the TOPTICA lasers are extremely robust and long-living, but also more expensive. The first-generation mesoSPIM had dedicated TOPTICA laser combiner (iChrome MLE ultra-stable laser combiner) for each excitation arm, which is very costly, but also very resilient. In addition, their 561 nm FDDL (frequency-doubled diode laser) has several advantages over conventional DPSS lasers. Depending on the budget and number of channels required, a mesoSPIM can be equipped with either of the following options from iChrome family (prices only estimated, assuming 4 colors, power output measured after fiber):

• 2x CLE-20 combiners (up to 4 colors, 561 FDDL lasers, 20 mW per channel - which is a bit lower than typically needed), total ~50k EUR.
• 1x FLE (up to 7 colors, dual fiber output, 50/50 power splitting, 561 FDDL), total ~53k EUR
• 2x CLE-50 combiners (same as CLE-20, but power up to 50 mW), total ~62k EUR
• 2x MLE-HP (up to 4 colors, 100mW per channel, 561 DPSS plus AOM), total ~96k EUR

As always, ask the company representatives for in-depth information and actual quotes.

Omicron

Again, from our own experience, on some systems the 561nm DPSS lasers are not very long-lived, and can require a costly replacement after one or two years of use. However, we also know very stable Omicron SOLE combiners which work trouble-free for many years.

:bulb: If you have other first-hand experience as a mesoSPIM owner, either positive or negative, feel free to share with us via email.