*3.2. The TOP Refractometer*

While the construction of the SOP, and the role of its various comprised parts, have previously been described in some detail in the literature [**? ?** ], those of the TOP have not. Therefore, we describe those in more detail here. As is shown in Figure **??**, the TOP refractometer system was built within a 19-inch transportable rack. For sturdy transportation and best serviceability, the system has been divided into a number of subsystems. From top to bottom, they comprise a top unit (denoted the cavity unit) and seven subrack enclosures, denoted the modules (A–G), comprising a number of separate mechanical, optical, and electrical entities that play the same role in the system as the corresponding parts do in the SOP system [**? ?** ].

The cavity unit consists of a 60 × 60 cm breadboard that is firmly attached to the top of the rack. The DFPC Invar spacer sits at the center of this breadboard within an aluminum enclosure (oven) that is temperature controlled by four Peltier elements. The breadboard is, in turn, temperature regulated by a heat mat placed under it. Mounted to the breadboard, surrounding all components in this unit, there is a 60 × 60 × 25 cm aluminum framework with thermally isolated walls (shown in the figure).

Four pneumatic valves, used to control the flow of gas into and out of each cavity, are attached to the top of the oven. As is further described in Section **??**, two of these valves (denoted *VT*.1 and *VT*.3) are connected to the gas supply unit, and two (*VT*.2 and *VT*.4) are connected to a turbo pump. To provide an assessment of the reference pressure, a pressure gauge (denoted *GT*.2 in Section **??**) is mounted on the turbo line (in close proximity to the reference cavity).

The cavity unit also contains customized fiber collimators that mode match the light from the lasers into the cavities; mirrors that direct the light; and detectors (Thorlabs, PDA50B-EC), placed behind each cavity, that detect the light transmitted on resonance.

Module A comprises the gas inlet system, consisting of a mass flow controller and an electronic pressure controller (denoted MFC and EPC in Section **??**, respectively) that provide a continuous flushing of gas and regulation of the pressure; and a pressure gauge, *GT*.1, that provides a rough assessment of the pressure under scrutiny. It also contains a four slot compact DAC (CompactDAQ, National Instruments, cDAQ-9174) that holds an analogue input module (National Instruments, NI-9215) to monitor the feedback voltages sent to the lasers; a temperature input module (National instruments, NI-9216) to measure the Pt-100 readings; a voltage output module (National Instruments, NI-9263) to give feed back to the Peltier drivers; and a digital output module (National Instruments, NI-9474) to control the pilot valves (which also resides in Module A). The front panel is equipped with a VCR port to connect the device to be scrutinized by the TOP (the device under test, DUT).

The rear panel is equipped with 230, 24, and 12 V power supply inputs (in the leftmost part of the figure). Above these, there are two USB connectors to the cDAQ and the MFC/EPCs. At the center there are eight push-in 6 mm pneumatic fittings to provide pressurized air to the seven pilot valves and the gas to the supply unit. Above these, there are three D-sub connectors, which are used to connect the high pressure gauge to the vacuum gauge controller (Oerlikon-Leybold, Graphix Three); the cDAQ with the Peltier driver; and a fill pressure relay with the gas filling valve. In the rightmost part of this panel there are two gas connectors: one VCR that is connected to the valve system inside the cavity unit at the top of the rack, and one Swagelok connector that can be used for rough pumping of the gas system.

**Figure 2.** The TOP system seen from the front and rear. All lasers, electronics, and gas connections are placed within a 19-inch rack. On top of the rack, there is a 60 × 60 × 25 cm encapsulated box (denoted the cavity unit) that contains, as its base, an optical breadboard, on which the Invar-based DFPC is placed (in turn, encapsulated in an aluminum enclosure, denoted the "oven"). This unit also comprises four pneumatic valves that control the filling and emptying of gas in the cavity during the GAMOR-cycles (as can be seen in Figure **??**, placed on top of the oven); and collimators, mirrors, and detectors that couple light into the cavities and measure the transmittance. The rack contains thereafter, from the top to the bottom, seven modules, denoted A–G, containing vacuum connectors, a communication hub, fiber-optics, a frequency counter, two fiber lasers, and locking electronics. The rack stands on four wheels that allow the system to be easily moved within the laboratory.

Module B contains most of the optics, passive fiber optical components (e.g., circulators and isolators), and opto-electronics. The leftmost part of the front panel comprises the output from the beat detector and the input for the fibers from the lasers. The light that enters via the fibers is coupled into acousto optic modulators (AOM, AA Opto-Electronic, MT110-IR25-3FIO), after which it is coupled into 90/10 splitters. The light in the 10% outputs of the two splitters is coupled to the beat detector (Thorlabs, PDA8GS) via a 50/50 combiner. The light in the 90% outputs is coupled into electro-optic modulators (EOM, General Photonics, LPM-001-15) for the production of sidebands for the Pound– Drever–Hall locking. The light fields are then coupled into circulators via isolators (to prevent back reflections to the EOM). The forward output of each circulator is coupled via a fiber to the collimator for further passage into the cavity unit, and their rear outputs, which monitor the reflections from the cavities, are connected to reflection detectors (Thorlabs, PDA10CE-EC). The front panel is also equipped with five BNC-ports that are connected to the transmission and reflection detectors of the system. The fifth of these is used as a trigger that enables an oscilloscope to be connected to the other ports for the alignment procedure of the free space optics in the cavity unit.

On the rear panel, nine SMA-connectors are positioned to the left, comprising the control signals for the EOM and AOM; the inputs from the transmission detectors (for the monitoring port on the front panel); and the outputs from the reflection detectors (for the feedback signal to the automatic locking unit). The ninth port is the trigger input from a digital laser locking module (Toptica, DigiLock 110). At the center are the circulator outputs, which are connected to the collimators in the cavity unit, and a USB port (not connected).

Module C holds a frequency counter (Aim-TTi, TF960) that measures the beat frequency detected by the beat detector (positioned in module B) and the vacuum gauge controller that is used to monitor the pressure gauges within the system (the *GT*.1 monitoring the pressure under scrutiny and *GT*.2 recording the reference pressure). The frequency counter and gauge controller are digitally controlled and monitored but can also be reached manually, as their fronts are shown on the front panel of this module. The back panel comprises three D-sub connectors, which are used to connect the two pressure gauges and the fill pressure relay, and two USB-ports, which are the communication interface for the frequency counter and the vacuum gauge controller.

Module D comprises various 12 and 24 V power supplies; the custom made voltage controlled oscillators, which regulate the AOMs in module B; servo circuits for the locking of the lasers to the cavity modes; and the control unit for the heat mat (JUMO, diraTRON 108) that regulates the breadboard under the cavity unit, seen at the center of the front panel. The front panel is also equipped with two SMA ports for monitoring of the VCOs.

In the center of the back panel, the output for the driving current of the Peltier elements and the heat mat can be found. The rightmost part of the back panel comprises the inputs and outputs for the VCO and Laser PZT voltages.

Module E is a power distribution unit, in which the main 230 V input is split into nine 230 V outputs for power distribution to each subsystem.

Module F consists of two Er-doped fiber lasers (EDFL, NTK, Koheras Adjustik E15) that produce the light that is coupled, through fibers, to module C.

Module G, on the very bottom, contains the two digital laser locking modules (Toptica, DigiLock110) used for the automatic locking procedure of the lasers to their respective cavity.

Finally, at the bottom right on the back side of the rack there is a USB-hub that connects various electronics to a laptop accompanying the TOP system. The system is controlled by the laptop that, through the use of custom made LabVIEW software, gathers all data required for analysis.

#### *3.3. The Gas Handling System for the Dual Refractometry System Used in This Work*

Figure **??** shows a schematic view of how the two refractometers are connected to the gas system comprising a common gas supply and distribution system.

In the figure, the colored lines represent gas tubes where the red color relates to low pressures while the blue represents high pressures. To fill the system with gas, the gas supply system, consisting of a mass flow controller (MFC, Bronkhorst, FG-201CV) and an electronic pressure controller (EPC Bronkhorst, P-702CV), is connected to a supply (in this work *N*2). In the volume to the left of valve *VS*,5, gas constantly circulates to prevent contamination build up.

When the refractometers are to be filled with gas, the valves *VS*/*T*.5 and *VS*/*T*.1 are opened. Valve *VS*,5 is opened and closed by a relay controlled by switching the set-point of the vacuum gauge controller. The input for the set-point is the pressure measured by gauge *GS*,1 (Oerlikon-Leybold, CTR 101 N 1000 Torr) in the SOP-refractometer and a set pressure chosen "close" to the nominal set value of the DWPG (as given by Equation (**??**)). This setup means that the gas system will be re-pressurized whenever the pressure drops below the chosen set pressure. After the re-pressurization the DWPG will automatically regulate the pressure to its set-pressure. During the gas filling and stabilization stage, the valves *VS*/*T*.2 and *VS*/*T*.3 are closed, and the valves *VS*/*T*.4 are open, resulting in evacuation (close to vacuum) of the reference cavities in both of the refractometers, represented by the red gas lines. When both measurement cavities are to be evacuated, the valves *VS*/*T*.1 are closed and the valves *VS*/*T*.2 are opened, leading to the evacuation of all cavities.

The gas lines are not depicted at an appropriate scale; counted from the common tee (depicted above valve *VS*,5) and the gas molecular turbo pump, the gas lines to the TOP are significantly longer than the corresponding ones to the SOP.

**Figure 3.** Schematic view of the gas delivery system. The components are described by the legend in the lower right corner. The setup is divided into three sub-systems, viz., the SOP, the TOP, and the DWPG. The SOP, presented in the upper left corner, regulates and controls the gas filling unit, consisting of the MFC and EPC. It also controls the primary fill valve, *VS*,5. The valves *VS*.1−4, which control the filling and evacuation of the cavities, are opened or closed in a given sequence. This subsystem also comprises two pressure gauges: *GS*.1, which monitors the high pressure side; and *GS*.2, which measures the residual pressure on the low pressure side. The TOP, which is displayed in the upper right corner, applies the same logic as the SOP system to its valves (*VT*.1−4) and gauges (*GT*.1−2). This subsystem can additionally be connected to or disconnected from the gas delivery system by use of valve *VT*.5. Finally, the DWPG is presented in the lower left corner. This is connected or disconnected to the rest of the system by the valve *VR*.1. In this presentation, the systems are displayed with the filling/connecting valves, *VS*.5, *VS*.1, *VT*.5, *VT*.1, and *VR*.1 open. This implies that the measurement cavities are filled by the pressure set by the DWPG (represented by the blue gas lines). In addition, the valves *VS*.4 and *VT*.4 are open to allow for an evacuation of the reference cavities (represented by the red gas lines). The measurement cycle is followed by an evacuation state in which the valves *VS*.1 and *VT*.1 are closed and *VS*.2 and *VT*.2 are opened, which evacuates all cavities.
